constify exec_file_attach

This constifies exec_file_attach and updates the rest of gdb.

Insight will need some minor tweaks after this, though it's worth
noting that I think all that hook stuff can actually just go away.  I
sent a patch to this effect once, but since the Insight source
repository situation isn't currently resolved there wasn't a
convenient way to test it.

2014-07-30  Tom Tromey  <tromey@redhat.com>

	* corefile.c (hook_type, call_extra_exec_file_hooks)
	(specify_exec_file_hook): Constify.
	* exec.c (exec_file_attach): Make "filename" const.
	* gdbcore.h (deprecated_exec_file_display_hook)
	(specify_exec_file_hook, exec_file_attach): Constify.
	* main.c (captured_main): Use catch_command_errors_const.

gdb/exec.c

@@ -154,7 +154,7 @@ exec_file_clear (int from_tty)
    we're supplying the exec pathname late for good reason.)  */
 
 void
-exec_file_attach (char *filename, int from_tty)
+exec_file_attach (const char *filename, int from_tty)
 {
   struct cleanup *cleanups;