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.

	* fileio.c (find_file_handler): Rename this to
	Ffind_file_name_handler, and make it visible to lisp.  Add a QUIT
	to the loop which scans file-name-handler-alist.  All uses
	changed.
	(syms_of_fileio): Mention this new function in the docstring for
	Vfile_name_handler_alist.  defsubr Sfind_file_name_handler.
	* lisp.h (Ffind_file_name_handler): Added extern declaration.
	* dired.c: All uses of find_file_handler changed here too.

	* fileio.c (syms_of_fileio): Add staticpros for Qexpand_file_name,
	Qdirectory_file_name, Qfile_name_directory,
	Qfile_name_nondirectory, Qfile_name_as_directory.