MFC r363277-r363283

r363277:
Only use the use_inet6 variable when INET6 is a build option.

This is a prerequisite to upcoming argument processing cleanups which
will resolve consistency as was done with ippool previously.

PR: 247952

r363278:
fr_family (the protocol family) must be AF_INET or AF_INET6, as in
the kernel, not an arbitrary 4 or 6.

This only affected printing ipfilter stats and rules from a kernel
dump. (This is currently undocumented.)

PR: 247952

r363279:
Historically ipfstat listings and stats only listed IPv4 or IPv6 output.
ipfstat would list IPv4 outputs by default while -6 would produce IPv6
outputs. This commit combines the ipfstat -i and -o outputs into one
listing of IPv4 and IPv6 rules. The -4 option lists only IPv4 rules
(as the default before) while -6 continues to list only rules that affect
IPv6.

PR: 247952
Reported by: joeb1@a1poweruser.com

r363280:
ipfstat -t defaults to IPv4 output. Make consistent with ipfstat -i
and ipfstat -o where without an argument IPv4 and IPv6 states are
shown. Use -4 and -6 to limit the display to IPv4 or IPv6 respectively.

PR: 247952

r363281:
Make ipfstat -t header generic when IPv4 and IPv6 output are
displayed in the same display.

PR: 247952

r363282:
The output from usage() need not contain usage for -t when STATETOP
is not compiled in.

PR: 247952