Make extract_arg return a std::string

Make extract_arg return a std::string

Change extract_arg to return a std::string and fix up all the users.
I think string is mildly better than unique_xmalloc_ptr<char>, when
possible, because it provides a more robust API.

I changed the error messages emitted from find_location_by_number to
avoid either writing to a string or an extra allocation; this can be
changed but I thought that the new message was not any less clear.
You can see an example in the testsuite patch.

ChangeLog
2017-09-11  Tom Tromey  <tom@tromey.com>

	* demangle.c (demangle_command): Update.
	* breakpoint.c (disable_command): Update.
	(enable_command): Update.
	(find_location_by_number): Make "number" const.  Use
	get_number_trailer.
	* cli/cli-utils.c (extract_arg): Return std::string.
	* probe.c (parse_probe_linespec): Update.  Change types.
	(collect_probes): Take string arguments.
	(parse_probe_linespec): Likewise.
	(info_probes_for_ops): Update.
	(enable_probes_command): Update.
	(disable_probes_command): Update.
	* break-catch-sig.c (catch_signal_split_args): Update.
	* mi/mi-parse.c (mi_parse): Update.

testsuite/ChangeLog
2017-09-11  Tom Tromey  <tom@tromey.com>

	* gdb.base/ena-dis-br.exp (test_ena_dis_br): Update test.