Fix Fdirectory_append check for whether strings have to be converted

Fix Fdirectory_append check for whether strings have to be converted

* src/coding.c (string_ascii_p): Make it non-static.

* src/fileio.c (Fdirectory_append): Fix check for whether we need
to convert to multibyte.

* src/fns.c (string_ascii_p): Remove copy.

* src/lisp.h: Declare string_ascii_p.