Give subprocess creation a way to find a valid current directory

Give subprocess creation a way to find a valid current directory
	for subprocesses when the buffer's default-directory is a handled
	name.
	* fileio.c (Funhandled_file_name_directory): New function.
	(Qunhandled_file_name_directory): New file-name-handler operation.
	(syms_of_fileio): Defsubr Sunhandled_file_name_directory, and
	initialize and staticpro Qunhandled_file_name_directory.
	* callproc.c (Fcall_process): Call Funhandled_file_name_directory
	on the buffer's default directory.  Do it earlier in the function
	so there's less to GCPRO.
	* process.c (create_process): Don't check the validity of the
	buffer's default directory here...
	(Fstart_process): Instead, do it here; if we call
	Funhandled_file_name_directory here, there's less GCPROing to do.