[binutils-gdb] Change gdb_realpath_keepfile to return a unique_xmalloc_ptr

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT 4971c9a74b47103582834e46d0185390379e60b3 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: 4971c9a74b47103582834e46d0185390379e60b3

Change gdb_realpath_keepfile to return a unique_xmalloc_ptr

This changes gdb_realpath_keepfile to return a unique_xmalloc_ptr, and
fixes up the callers.

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

	* utils.c (gdb_realpath_keepfile): Return a
	gdb::unique_xmalloc_ptr.
	* exec.c (exec_file_attach): Update.
	* utils.h (gdb_realpath_keepfile): Return a
	gdb::unique_xmalloc_ptr.