Change openp et al to use a unique_xmalloc_ptr

Change openp et al to use a unique_xmalloc_ptr

This changes openp, source_full_path_of, and find_and_open_source to
take a unique_xmalloc_ptr, rather than a char*, as an outgoing
argument type.  This simplifies the API, ownership-wise, and allows
for the removal of some cleanups.

gdb/ChangeLog
2018-02-14  Tom Tromey  <tom@tromey.com>

	* symfile.c (symfile_bfd_open): Update.
	* source.h (openp, source_full_path_of, find_and_open_source):
	Change argument type to unique_xmalloc_ptr.
	* source.c (openp): Take a unique_xmalloc_ptr.
	(source_full_path_of, find_and_open_source): Likewise.
	(open_source_file, symtab_to_fullname): Update.
	* solist.h (struct target_so_ops) <find_and_open_solib>: Take a
	unique_xmalloc_ptr.
	* solib.c (solib_find_1): Use unique_xmalloc_ptr.
	(exec_file_find): Update.
	* psymtab.c (psymtab_to_fullname): Update.
	* nto-tdep.h (nto_find_and_open_solib): Update.
	* nto-tdep.c (nto_find_and_open_solib): Change temp_path to a
	unique_xmalloc_ptr.
	* exec.c (exec_file_attach): Update.
	* dwarf2read.c (try_open_dwop_file): Use unique_xmalloc_ptr.
	* cli/cli-cmds.c (find_and_open_script): Use unique_xmalloc_ptr.