Change gdb_abspath to return a unique_xmalloc_ptr

Change gdb_abspath to return a unique_xmalloc_ptr

This changes gdb_abspath to return a unique_xmalloc_ptr, and fixes up
the callers.  This allows the removal of a cleanup, and also puts
ownership rules into the API, where they belong.

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

	* compile/compile.c (compile_file_command): Use
	gdb::unique_xmalloc_ptr, std::string.
	* utils.c (gdb_abspath): Change return type.
	* source.c (openp): Update.
	* objfiles.c (allocate_objfile): Update.
	* main.c (set_gdb_data_directory): Update.
	* utils.h (gdb_abspath): Return a gdb::unique_xmalloc_ptr.