Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Now actually implement reading/refreshing/returning data from the pfTablesAddrTable
and modify the BEGEMOT-PF-MIB to add support for IPV6 address' statistics in the PF
tables via pfTablesAddrNetType and pfTablesAddrNet. While here, upgrade the
pf_tree.def file to the new format that includes enumerated values. Also make sure
to return SNMP_ERR_NOSUCHNAME for ALTQ objects, if ALTQ is disabled, so that the agent
will know to skip the pfAltq subtree when servicing GETNEXT requests from SNMP clients
(otherwise snmpwalk on begemotPf would stop at the pfAltq subtree with bsnmpd returning
SNMP_ERR_GENERR).

The last big commit: let usr.sbin/ use WARNS=6 by default.

Use the new bsd.snmpmod.mk makefile to build the modules. The -I pointers
into the contrib directory are still necessary for some of the Makefiles,
because the C-sources there use non-canonical includes ("" includes) to get
at the header files.

Add snmp_pf, a bsnmpd module for monitoring (not managing!) pf firewalls.
Currently implemented are most things related to states and interfaces, todo
are address tables and altq. I've tried to keep this roughly in sync with a
project implementing this for net-snmpd.

Reviewed by: harti, dhartmei
MFC after: 1 month