Change gdb_realpath to return a unique_xmalloc_ptr

Change gdb_realpath to return a unique_xmalloc_ptr

This changes gdb_realpath to return a unique_xmalloc_ptr and fixes up
the callers.  This allows removing some cleanups.  This change by
itself caused xfullpath.exp to fail; and attempting to fix that ran
into various problems (like .get() being optimized out); so this patch
also rewrites xfullpath.exp to be a C++ selftest instead.

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

	* exec.c (exec_file_attach): Update.
	* linux-thread-db.c (try_thread_db_load): Update.
	* guile/scm-safe-call.c (gdbscm_safe_source_script): Update.
	* utils.c (gdb_realpath): Change return type.
	(gdb_realpath_keepfile): Update.
	(gdb_realpath_check_trailer, gdb_realpath_tests): New functions.
	(_initialize_utils): Register the new self test.
	* source.c (openp): Update.
	(find_and_open_source): Update.
	* nto-tdep.c (nto_find_and_open_solib): Update.
	* main.c (set_gdb_data_directory): Update.
	(captured_main_1): Update.
	* dwarf2read.c (dwarf2_get_dwz_file): Update
	(dw2_map_symbol_filenames): Update.
	* auto-load.c (auto_load_safe_path_vec_update): Update.
	(filename_is_in_auto_load_safe_path_vec): Change type of
	"filename_realp".
	(auto_load_objfile_script): Update.
	(file_is_auto_load_safe): Update.  Use std::string.
	* utils.h (gdb_realpath): Return a gdb::unique_xmalloc_ptr.

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

	* gdb.gdb/xfullpath.exp: Remove.