Update: misc bugfixes and dummynet(4) integration

Packet filtering by ipfw(4) will be enabled by setting

	# sysctl net.mpls.pfil_ipfw=1

or by

	# sysctl net.mpls.pfil_enable=1

for using other software accessing pfil(9).
	Service Access Point (SAP) for packet filtering by
pfil(9) implemented Inspection Access Point (IAP) is
implemented by mpls_pfil() which is derived from
bridge_pfil() (see net/if_bridge.c) by reusing code
sections. Why? Due to simplicity in avoidance to reinvent
the wheel.

Overview of given SAPs providing IAPs:

 A    to upper layers   V
 |                      |
 +----------->----------+
 A                      |
 |                      V
 + ip[6]_input()        + ip[6]_output()
 |                      |
 |                      |
 |  + mpls_forward() +  |
 | /                  \ |
 |/                    \|
 + mpls_input(), IAP    + mpls_output(), IAP
 A                      |
 |                      |  either
 |                      |
 |                      |   net.mpls.pfil_ipfw=1
 |                      |
 |                      |  or
 |                      |
 |                      |   net.mpls.pfil_enable=1
 |                      |
 |                      V
 + ether_demux()        + ether_output_frame()
 |                      |
 |                      |
 +-->+ bdg_forward() +--+
 A                      |
 |      to devices      V

Bugfixes:
---------

 o Misc bugfixes targeting if_bridge(4)
 o Cleanup in mpls_rtalert.c

Next issues:
------------

Review targeting exception handling, due to exceeding TTL.
Misc. bgfixes and testing of dummynet(4) and pfil(9)
related issues.