Log message:
Add syncffsd

syncffsd uses rsync(1) to replicate a directory tree to another host.
This involves detecting changes, something NetBSD is not well prepared to
handle. The kqueue(2) interface can detect changes, but it needs to use a
file descriptor for every node monitored, and this does not scales very
well for huge directories trees.

syncffsd attempts to improve the situation until kqueue(2) gets better,
by using FFS superblock's fs_time field, which is updated on every
filesystem change. This lets syncffsd detects a change immediatly if the
filesystem is mounted synchronous, or after a few seconds if mounted with
WAPBL(4) enabled.

Once a change is detected, the relevant nodes are found by walking source
looking for files that changed since the previous filesystem modification.
The resulting nodes are deduped (i.e.: we do not retain a node in
a directory if the directory itself changed), and are fed to rsync(1).