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.c

@@ -354,6 +354,7 @@ ssize_t __msg_build_method_call(uint8_t *buf, size_t cap,
 				    const char *path,
 				    const char *interface,
 				    const char *member,
+				    const char *destination,
 				    const char *signature,
 				    uint32_t body_len)
 {
@@ -369,6 +370,9 @@ ssize_t __msg_build_method_call(uint8_t *buf, size_t cap,
 		return -1;
 	if (put_field_string(buf, cap, &off, LINK_HDR_MEMBER, 's', member) < 0)
 		return -1;
+	if (destination &&
+	    put_field_string(buf, cap, &off, LINK_HDR_DESTINATION, 's', destination) < 0)
+		return -1;
 	if (signature && *signature &&
 	    put_field_string(buf, cap, &off, LINK_HDR_SIGNATURE, 'g', signature) < 0)
 		return -1;