It might not be feasible for users to start passt-repair after passt
is started, on a migration target, but before the migration process
starts.

For instance, with libvirt, the guest domain (and, hence, passt) is
started on the target as part of the migration process. At least for
the moment being, there's no hook a libvirt user (including KubeVirt)
can use to start passt-repair before the migration starts.

Add a directory watch using inotify: if PATH is a directory, instead
of connecting to it, we'll watch for a .repair socket file to appear
in it, and then attempt to connect to that socket.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

...perhaps I should adopt the healthy habit of actually reading
headers instead of using my mental copy.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

A privileged helper to set/clear TCP_REPAIR on sockets on behalf of
passt. Not used yet.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>