MPLS: mpls_pfil: SAP maps to pfil(9) based IAP!!!

Definition of Service Access Point (SAP) maps to by pfil(9)
implemented Inspection Access Point (IAP) targeting (twice)
mpls_forward() implemented by mpls_pfil().

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).

Due to simplicity, I've decided to implement mpls_pfil()
as derived from bridge_pfil() by reusing of code sections,

Why? Often, it is a common mistake to reinvent the wheel,
when the wheel still exists.

But this stuff is partially tested!!! Testing is now in
progress... please stand by.

Bugfixes:
---------

Forgotten m_tag_prepend() added in bridge_pfil().

Next implementation task:
-------------------------

Further, dummynet(4) integration and bugfixes targeting
MPLS ARP and route handling maps to reserved labels.