Misc. bugfixes. due to route(8) and netmpls/mpls_rmx.c.

In function mpls_xconnect_fib() missing (forgotten) RTF_MPE enabled
in conjunction set RTF_PUSH operation. Defined RTF_MPE flag ensures
such that ingress MPLS label mapping is valid. Otherwise, by ip_output()
generated PDU in FEC maps to granular MPLS ingress label won't be tagged
by out-seg.

Introduced from K_PUSH derived K_STACK command maps to RTF_STACKING flag
for stacking in route(8).

Let us assume misc LSR with em1: 10.0.1.1 shares same broadcast media
with NH 10.0.1.2 and FEC 192.168.1.4 by

  # route add 192.168.1.4 10.0.1.2

then

  # route add 192.168.1.4 -stack 100 -mpls 200

dictates LSR by x-connect to push out-seg 200 on top of mpls label stack,
if and only if LSR receives Frames containing by MPLS label 100 PDUs.

Btw, implementation behind K_STACK command is partially incomplete,
see i. e. in mpls_forward() in netmpls/mpls_input.c.