L1CTL: Add message segmentation helper via getMsgLen()

As L1CTL is using a stream socket, we need to give the UNIX_DOMAIN
port some clue as to where our L1CTL message boundaries are in the
stream.

This requires a patched UNIX_DOMAIN_SOCKETasp test port with the
following commit applied:

commit 655cb4ab2ac006b3a73d1b679c83081d9743410a
Author: Harald Welte <laforge@gnumonks.org>
Date:   Sun Feb 25 23:25:46 2018 +0100

    Add "getMsgLen" function similar to IPL4asp_PT

Change-Id: Iab33f57cb4311180e521a76307a552d16287b062
10 files changed