gssd: base the size of the fd array on the RLIMIT_NOFILE limit.

We have previously raised the size of the 'pollarray' once (32 -> 256)
and I have had another request to make it bigger.
Rather than changing the hard-coded value, make it depend on
RLIMIT_NOFILE.  This is an upper limit on the size of the array
that can be passed to poll() anyway.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>