Return std::string from perror_string

Return std::string from perror_string

Change perror_string to return a std::string, removing a cleanup in
the process.

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

	* utils.c (perror_string): Return a std::string.
	(throw_perror_with_name, perror_warning_with_name): Update.