Fix djgpp gdb build

- A few missing casts required by C++, resulting in:

   ../../src/gdb/ser-go32.c:795:21: error: invalid conversion from 'const void*' to 'const char*' [-fpermissive]

   etc.

 - dos_noop has an incompatible prototype with struct serial_ops's
   setparity, resulting in:

    ../../src/gdb/ser-go32.c:874:1: error: invalid conversion from 'int (*)(serial*)' to 'int (*)(serial*, int)' [-fpermissive]

   (I thought of calling the ser-base.c default methods, but djgpp
   doesn't include ser-base.c in the build.)

gdb/ChangeLog:
2016-07-21  Pedro Alves  <palves@redhat.com>

	* go32-nat.c (go32_create_inferior): Add cast.
	* ser-go32.c (dos_noop): Delete.
	(dos_flush_output, dos_setparity, dos_drain_output): New
	functions.
	(dos_write): Add cast.
	(dos_ops): Use dos_flush_output, dos_setparity and
	dos_drain_output.
	* top.c (do_chdir_cleanup): Add cast.

gdb/top.c

@@ -470,7 +470,7 @@ void (*pre_init_ui_hook) (void);
 static void
 do_chdir_cleanup (void *old_dir)
 {
-  chdir (old_dir);
+  chdir ((const char *) old_dir);
   xfree (old_dir);
 }
 #endif