daemon: use callback to build interpolated path

daemon: use callback to build interpolated path

Provide a callback function for strbuf_expand() instead of using the
helper strbuf_expand_dict_cb().  While the resulting code is longer, it
only looks up the canonical hostname and IP address if at least one of
the placeholders %CH and %IP are used with --interpolated-path.

Use a struct for passing the directory to the callback function instead
of passing it directly to avoid having to cast away its const qualifier.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>