ipfilter: Make LARGE_NAT a tunable.

LARGE_NAT is a C macro that increases
NAT_SIZE from 127 to 2047,
RDR_SIZE from 127 to 2047,
HOSTMAP_SIZE from 2047 to 8191,
NAT_TABLE_MAX from 30000 to 180000, and
NAT_TABLE_SZ from 2047 to 16383.

These values can be altered at runtime using the ipf -T command however
some adminstrators of large firewalls rebuild the kernel to enable
LARGE_NAT at boot. This revision adds the tunable net.inet.ipf.large_nat
which allows an administrator to set this option at boot instead of build
time. Setting the LARGE_NAT macro to 1 is unaffected allowing build-time
users to continue using the old way.