Backport: relayd: rename fd-cap parameter to fd-pool-size

Backport: relayd: rename fd-cap parameter to fd-pool-size

Rename the fd-cap parameter and change its default behaviour.
The minimum number of file descriptor is raised to 100 and a
"reserve" amount of 10 fds is allowed to accomodate transient
fd uses that can't be tracked by the relay daemon.

The --fd-pool-size will accept parameters in the
[100, fileno soft limit] interval.

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