* nlmread.c (nlm_symtab_read): Clean up a bit.

* (nlm_symfile_read):  Record bounds of main() so that backtrace
	command will know where to stop.
	* objfiles.c (objfile_relocate):  Relocate entry point/func info
	for backtrace as well.
	* objfiles.h:  Define values for invalid PCs for entry point info.
	* symfile.c (init_entry_point_info):  Initialize invalid values
	with aforementioned macros.
	* config/alpha/tm-alphanw.h:  Turn on FRAME_CHAIN_VALID_ALTERNATE
	to cause backtrace to stop when it gets back to main().
	* config/i386/tm-i386nw.h:  Ditto.

gdb/objfiles.h

@@ -93,6 +93,8 @@ struct entry_info
 
   CORE_ADDR entry_point;
 
+#define INVALID_ENTRY_POINT (~0) /* ~0 will not be in any file, we hope.  */
+
   /* Start (inclusive) and end (exclusive) of function containing the
      entry point. */
 
@@ -110,6 +112,15 @@ struct entry_info
   CORE_ADDR main_func_lowpc;
   CORE_ADDR main_func_highpc;
 
+/* Use these values when any of the above ranges is invalid.  */
+
+/* We use these values because it guarantees that there is no number that is
+   both >= LOWPC && < HIGHPC.  It is also highly unlikely that 3 is a valid
+   module or function start address (as opposed to 0).  */
+
+#define INVALID_ENTRY_LOWPC (3)
+#define INVALID_ENTRY_HIGHPC (1)
+
 };