libink/dbus: identify the caller behind a broker

On the local bus SO_PEERCRED says who is calling and the kernel is the
one saying it.  Behind a broker one connection carries every caller,
so that credential describes dbus-daemon and nothing else, and every
privileged method was refused there, root included.

Ask the bus driver instead.  libink parks the call and hands us the
sender; we ask GetConnectionUnixUser and answer when the reply lands,
through the same event loop as everything else.  Nothing blocks:
blocking in PID 1 is why libuEv exists.  That needs calls libink can
make on a connection it already has, so it gained those too.

Answers are cached, since a bus never reuses a unique name while it
runs.  Not across a restart though: a new dbus-daemon numbers from
scratch and :1.7 becomes somebody else, so the cache goes when the
broker does.  A sender name too long to key on is refused rather than
truncated, two callers sharing a truncated key would share an
identity.

Privilege is no longer uid 0 alone.  The socket is already owned by
the --with-group group, so refusing its members every method that
changes anything left a wheel user able to open the bus and unable to
reboot.  Both gates now say the same thing.

Group membership needs NSS, which the C library loads with dlopen(),
so the lookup is compiled out where Finit is built to link statically.
That leaves such a build root-only, which is worth saying out loud
rather than leaving to be discovered.

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

libink/proto.h

@@ -60,6 +60,11 @@ struct link_msg {
  * needed, -1 on malformed input. */
 ssize_t __msg_parse(const uint8_t *buf, size_t len, struct link_msg *out);
 
+/* Project a parsed message onto the public reply view.  Shared by the
+ * synchronous client and the connection-side reply routing so the two
+ * cannot drift as link_reply_t grows. */
+void __msg_to_reply(link_reply_t *r, const struct link_msg *m);
+
 /* Compute the on-wire size of a future message header given the
  * fields we'd populate.  Used to size send buffers. */
 size_t __msg_header_size(const struct link_msg *m);