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
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D53284