Avoid extra calls to strlen in filesystem I/O routines.

Avoid extra calls to strlen in filesystem I/O routines.
* fileio.c (Fexpand_file_name): Avoid calls to strlen if
the length of 'newdir' is known or may be precalculated.
(file_accessible_directory_p): Prefer to pass Lisp_Object,
not 'char *', and so use precalculated length.
(Ffile_accessible_directory_p):
* callproc.c (encode_current_directory, init_callproc):
* charset.c (init_charset):
* lread.c (load_path_check, load_path_default): Adjust users.
* lisp.h (file_accessible_directory_p): Tweak prototype.