-Wwrite-strings: Some constification in gdb/breakpoint.c

The main motivation here is avoiding having to write a couple casts
like these:

     if (!arg)
  -    arg = "";
  +    arg = (char *) "";

in catch_exception_command_1 and catch_exec_command_1.

That requires making ep_parse_optional_if_clause and
check_for_argument take pointers to const strings.  I then tried
propagating the resulting constification all the way, but that was
spiraling out of control, so instead I settled for keeping const and
non-const overloads.

gdb/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

	* break-catch-throw.c (handle_gnu_v3_exceptions): Constify
	'cond_string' parameter.
	(extract_exception_regexp): Constify 'string' parameter.
	(catch_exception_command_1): Constify.
	* breakpoint.c (init_catchpoint)
	(create_fork_vfork_event_catchpoint): Constify 'cond_string'
	parameter.
	(ep_parse_optional_if_clause, catch_fork_command_1)
	(catch_exec_command_1): Constify.
	* breakpoint.h (init_catchpoint): Constify 'cond_string'
	parameter.
	(ep_parse_optional_if_clause): Constify.
	* cli/cli-utils.c (remove_trailing_whitespace)
	(check_for_argument): Constify.
	* cli/cli-utils.h (remove_trailing_whitespace): Constify and add
	non-const overload.
	(check_for_argument): Likewise.

gdb/cli/cli-utils.c

@@ -238,8 +238,8 @@ number_is_in_list (const char *list, int number)
 
 /* See documentation in cli-utils.h.  */
 
-char *
-remove_trailing_whitespace (const char *start, char *s)
+const char *
+remove_trailing_whitespace (const char *start, const char *s)
 {
   while (s > start && isspace (*(s - 1)))
     --s;
@@ -288,7 +288,7 @@ extract_arg (char **arg)
 /* See documentation in cli-utils.h.  */
 
 int
-check_for_argument (char **str, char *arg, int arg_len)
+check_for_argument (const char **str, const char *arg, int arg_len)
 {
   if (strncmp (*str, arg, arg_len) == 0
       && ((*str)[arg_len] == '\0' || isspace ((*str)[arg_len])))