mpls_rt_output_fib() refactored.

Current state:
 o Building LSP and tx / rx MPLS Labeled PDU - works.
 o Exceptionhandling due to exceeding TTL (IPv4) - works.
 o To configure physical interfaces to operate as PE - works.
 o tx / rx of frames (PWE2) or dgrams by if_mpe(4) - works.
 o shortcut - works.

Next schedeuled tasks are:
 -> rt_msg handling
 -> IPv6 support
 -> improvements due to MPLS Label binding at PE
 -> refactoring ifconfig related stuff (e.g. mpls_control(), ...)
 -> correct implementation of enabling / disabling MPLS
 	at interface layer
 -> bugfixes

Outstanding tasks, e. g.:
 -> statistics / SNMP stuff
 -> handling of reserved labels (add / removal) via rtsock
 -> CW handling and policies due to VPS
 -> QoS Bits
 -> ...