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

-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.