libink: a message bus is not a peer

libink was written against the only bus it had, its own, where the
peer on the other end is the client.  A broker is not: it routes for
senders it names itself, expects a DESTINATION on anything addressed
through it, and answers on its own schedule rather than next.

Runlevels go on the wire as S and N rather than the digits Finit
keeps internally, since that is what a caller outside Finit means by
one.

The library stays a convenience library, linked into finit and
initctl and installed nowhere: the ABI promise waits until libink is
its own project.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

libink/proto.h

@@ -89,12 +89,14 @@ ssize_t __msg_build_signal(uint8_t *buf, size_t cap,
 			      const char *member,
 			      const char *signature, uint32_t body_len);
 
-/* Build a method-call header (client side). */
+/* Build a method-call header (client side).  `destination` is NULL
+ * when no broker routes the message. */
 ssize_t __msg_build_method_call(uint8_t *buf, size_t cap,
 				    uint32_t serial,
 				    const char *path,
 				    const char *interface,
 				    const char *member,
+				    const char *destination,
 				    const char *signature,
 				    uint32_t body_len);