socket-util: add new connect_unix_path() helper

This is a short helper for connecting to AF_UNIX sockets in the file
system. It works around the 108ch limit of sockaddr_un, and supports
"at" style fds.

This doesn't come with a test of its own, but the next patch will add
that.

src/basic/socket-util.h

@@ -332,3 +332,5 @@ int socket_get_mtu(int fd, int af, size_t *ret);
 
 /* an initializer for struct ucred that initialized all fields to the invalid value appropriate for each */
 #define UCRED_INVALID { .pid = 0, .uid = UID_INVALID, .gid = GID_INVALID }
+
+int connect_unix_path(int fd, int dir_fd, const char *path);