relayd: track directory handles through the fd-tracker

Track directory handles through the fd-tracker as unsuspendable
file descriptors. New fd-tracker utils are introduced to wrap
the creation and registration of the file descriptors to the
fd-tracker.

Note that file descriptors only need to be tracked when the
dirfd-backed implementation of the directory handles is used.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I754af1943f5d6f02a6219d48c8fc4b8106de1c13