MPLS: Update: forwarding.

Merged code from mpls_input() and rtalert_output() and moved into
mpls_forward().

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

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

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.

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