Use std::string thread.c

Use std::string thread.c

This changes a few spots in thread.c to use std::string, removing some
cleanups.

ChangeLog
2017-09-03  Tom Tromey  <tom@tromey.com>

	* thread.c (print_thread_info_1): Use string_printf.
	(thread_apply_command, thread_apply_all_command): Use
	std::string.