ipfilter: Add an htable max size tuneable.

Add an ipfilter runtime option (ipf -T) to adjust the default
maximum hash table size. Default it to 1024 entries. It will be
used by a subsequent commit to limit any damage due to excessively
large hash table input by the user.

Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D53284

(cherry picked from commit f3b94f47f55c502e8983f9bd294e963e75b2963a)