target_read_memory&co: no longer return target_xfer_status

Years ago, these functions used to return errno/EIO.  Later, through a
series of changes that intended to remove native/remote differences,
they ended up returning a target_xfer_status in disguise.

Unlike target_xfer_partial&co, the point of target_read_memory&co is
to either fully succeed or fail.  On error, they always return
TARGET_XFER_E_IO.  So there's no real point in casting the return of
target_read_memory to a target_xfer_status to pass it to memory_error.
Instead, it results in clearer code to simply decouple
target_read_memory&co's return from target_xfer_status.

This fixes build errors like this in C++ mode:

 ../../src/gdb/corefile.c: In function ‘void read_stack(CORE_ADDR, gdb_byte*, ssize_t)’:
 ../../src/gdb/corefile.c:276:34: error: invalid conversion from ‘int’ to ‘target_xfer_status’ [-fpermissive]
      memory_error (status, memaddr);
				   ^
 ../../src/gdb/corefile.c:216:1: error:   initializing argument 1 of ‘void memory_error(target_xfer_status, CORE_ADDR)’ [-fpermissive]

gdb/ChangeLog:
2015-10-27  Pedro Alves  <palves@redhat.com>

	* alpha-tdep.c (alpha_read_insn): Always pass TARGET_XFER_E_IO to
	memory_error.  Rename local 'status' to 'res'.
	* c-lang.c (c_get_string): Always pass TARGET_XFER_E_IO to
	memory_error.
	* corefile.c (read_stack, read_code, write_memory): Always pass
	TARGET_XFER_E_IO to memory_error.
	* disasm.c (dis_asm_memory_error): Always pass TARGET_XFER_E_IO to
	memory_error.  Rename parameter 'status' to 'err'.
	(dump_insns): Rename local 'status' to 'err'.
	* mips-tdep.c (mips_fetch_instruction): Rename parameter 'statusp'
	to 'errp'.  Rename local 'status' to 'err'.  Always pass
	TARGET_XFER_E_IO to memory_error.
	(mips_breakpoint_from_pc): Rename local 'status' to 'err'.
	* target.c (target_read_memory, target_read_raw_memory)
	(target_read_stack, target_read_code, target_write_memory)
	(target_write_raw_memory): Return -1 on error instead of
	TARGET_XFER_E_IO.
	* valprint.c (val_print_string): Rename local 'errcode' to 'err'.
	Always pass TARGET_XFER_E_IO to memory_error.  Update comment.

gdb/target.c

@@ -1380,7 +1380,7 @@ target_xfer_partial (struct target_ops *ops,
 
 /* Read LEN bytes of target memory at address MEMADDR, placing the
    results in GDB's memory at MYADDR.  Returns either 0 for success or
-   TARGET_XFER_E_IO if any error occurs.
+   -1 if any error occurs.
 
    If an error occurs, no guarantee is made about the contents of the data at
    MYADDR.  In particular, the caller should not depend upon partial reads
@@ -1399,7 +1399,7 @@ target_read_memory (CORE_ADDR memaddr, gdb_byte *myaddr, ssize_t len)
 		   myaddr, memaddr, len) == len)
     return 0;
   else
-    return TARGET_XFER_E_IO;
+    return -1;
 }
 
 /* See target/target.h.  */
@@ -1431,7 +1431,7 @@ target_read_raw_memory (CORE_ADDR memaddr, gdb_byte *myaddr, ssize_t len)
 		   myaddr, memaddr, len) == len)
     return 0;
   else
-    return TARGET_XFER_E_IO;
+    return -1;
 }
 
 /* Like target_read_memory, but specify explicitly that this is a read from
@@ -1446,7 +1446,7 @@ target_read_stack (CORE_ADDR memaddr, gdb_byte *myaddr, ssize_t len)
 		   myaddr, memaddr, len) == len)
     return 0;
   else
-    return TARGET_XFER_E_IO;
+    return -1;
 }
 
 /* Like target_read_memory, but specify explicitly that this is a read from
@@ -1461,14 +1461,14 @@ target_read_code (CORE_ADDR memaddr, gdb_byte *myaddr, ssize_t len)
 		   myaddr, memaddr, len) == len)
     return 0;
   else
-    return TARGET_XFER_E_IO;
+    return -1;
 }
 
 /* Write LEN bytes from MYADDR to target memory at address MEMADDR.
-   Returns either 0 for success or TARGET_XFER_E_IO if any
-   error occurs.  If an error occurs, no guarantee is made about how
-   much data got written.  Callers that can deal with partial writes
-   should call target_write.  */
+   Returns either 0 for success or -1 if any error occurs.  If an
+   error occurs, no guarantee is made about how much data got written.
+   Callers that can deal with partial writes should call
+   target_write.  */
 
 int
 target_write_memory (CORE_ADDR memaddr, const gdb_byte *myaddr, ssize_t len)
@@ -1479,14 +1479,14 @@ target_write_memory (CORE_ADDR memaddr, const gdb_byte *myaddr, ssize_t len)
 		    myaddr, memaddr, len) == len)
     return 0;
   else
-    return TARGET_XFER_E_IO;
+    return -1;
 }
 
 /* Write LEN bytes from MYADDR to target raw memory at address
-   MEMADDR.  Returns either 0 for success or TARGET_XFER_E_IO
-   if any error occurs.  If an error occurs, no guarantee is made
-   about how much data got written.  Callers that can deal with
-   partial writes should call target_write.  */
+   MEMADDR.  Returns either 0 for success or -1 if any error occurs.
+   If an error occurs, no guarantee is made about how much data got
+   written.  Callers that can deal with partial writes should call
+   target_write.  */
 
 int
 target_write_raw_memory (CORE_ADDR memaddr, const gdb_byte *myaddr, ssize_t len)
@@ -1497,7 +1497,7 @@ target_write_raw_memory (CORE_ADDR memaddr, const gdb_byte *myaddr, ssize_t len)
 		    myaddr, memaddr, len) == len)
     return 0;
   else
-    return TARGET_XFER_E_IO;
+    return -1;
 }
 
 /* Fetch the target's memory map.  */