relayd: initialize the global fd tracker from fd_cap option

relayd: initialize the global fd tracker from fd_cap option

Initialize the relay daemon's global fd tracker from the current
fd-cap configuration. This is in preparation for follow-up patches
which will introduce the tracking of the various file descriptor
backed resources to the relay daemon.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Iaabb80f1fb4d24345b750503c90388f94025159d