Change rewrite_source_path to return a unique_xmalloc_ptr

Change rewrite_source_path to return a unique_xmalloc_ptr

This changes rewrite_source_path to return a unique_xmalloc_ptr and
fixes up the callers.  This allows removing some cleanups.

ChangeLog
2017-08-22  Tom Tromey  <tom@tromey.com>

	* source.h (rewrite_source_path): Return a unique_xmalloc_ptr.
	* source.c (rewrite_source_path): Return a unique_xmalloc_ptr.
	(find_and_open_source, symtab_to_fullname): Update.
	* psymtab.c (psymtab_to_fullname): Update.