common: Use GPG_ERR_INV_VALUE instead of GPG_ERR_EINVAL.

* common/sysutils.c (gnupg_inotify_watch_socket): Return
GPG_ERR_INV_VALUE for a missing socket name and set proper error
source.

By using a different value we can easier see whether the error is due
to a system call or from GnuPG code.

• Signed-off-by: Werner Koch <wk@gnupg.org>