Ignore fd's larger than 65536 in varnishncsa

This makes varnishncsa behave the same way as varnishlog, and prevents
overly large memory allocations if it should see a bogus fd (shmlog
overrun?).

XXX: This will not fix varnishncsa for current trunk, as that is using
XIDs instead of FDs breaking some functionality in the tools. This
will be handled in the API rewrite.