credential-cache--daemon: refactor check_socket_directory

credential-cache--daemon: refactor check_socket_directory

This function does an early return, and therefore has to
repeat its cleanup. We can stick the later bit of the
function into an "else" and avoid duplicating the shared
part (which will get bigger in a future patch).

Let's also rename the function to init_socket_directory. It
not only checks the directory but also creates it. Saying
"init" is more accurate.

Signed-off-by: Jon Griffiths <jon_p_griffiths@yahoo.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>