Refactoring of mpls_control() interface and implementation of af_mpls.c.

NOTE: Don't panic!!!
NOTE:
NOTE: I know about the fact that it is bad style to commit
NOTE: incomplete things. I've done it to show some progress
NOTE: to resolve this issue (see above).
NOTE:
NOTE: At the moment, this stuff does not compiles (and contains
NOTE: some blatant desing flaws), because it remains in unfinished
NOTE: state, yet. Bat this will change soon (in a couple of hours).
NOTE:
NOTE: When I'm completed refactoring of mpls_control_interface,
NOTE: then I'm starting over to refactor if_mpe.c and mpls_output.c
NOTE: due to handling of PE Ingress Routes.  I've decided to refer
NOTE: information, which is needed to bind Ingress (or VP) label,
NOTE: at struct mpls_ifinfo {}.