keventd: defer pidfile until coldplug queue is drained

When started with -c (the default when keventd is the device manager),
gate the pidfile on the kernel's uevent_seqnum having been stable for
200ms.  Up to now ready signaling with the pidfile was done right after
coldplug() triggered the kernel to re-emit events, but before uev_run()
had drained any of them, so <pid/keventd> really only meant "listening
on netlink".

With the gate, services that depend on <pid/keventd> can now assume /dev
is populated and persistent symlinks are live.

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

doc/keventd.md

@@ -154,6 +154,12 @@ kernel to re-emit add events for all existing devices.
 
 This replaces the separate `coldplug` script previously used with mdev.
 
+When `-c` is used, keventd defers its pidfile (and the `pid/keventd`
+condition Finit asserts from it) until the coldplug event queue has
+been fully drained.  Services that depend on `<pid/keventd>` can
+therefore assume `/dev` is populated and persistent symlinks are live,
+without needing a separate `settle` step.
+
 
 Netlink Rebroadcast
 -------------------