relayd: add fd-cap option to limit the number of opened FDs

Add an --fd-cap option to the relay daemon in order to allow the
launch of the relay daemon with a maximal number of file descriptors
to be open at any given moment.

When the value of the --fd-cap parameter is left unset, the maximal
number of file descriptors is set to the system's NOFILE soft limit
(see GETRLIMIT(3P)).

A minimal number of file descriptors of 30 is imposed, mainly to
prevent absurd configurations (someone setting 1 fd) which would not
even allow one target to connect and stream traces. This also allows
a bit of leverage to open file descriptors that could be needed by
future changes without "breaking" an existing configuration.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ibaf49b25ccf3e3a8013115c9478744ca3646e306