mrpdclient lib use, step 1

I was on my way to convert the talker/listener pairs to use mrpdclient, but in the process introduced two API changes to it.

I converted to this new API the minimum to pass Travis: mrpctl, mrpl, mrpq, mrpValidate.

The 2 important commits are:

mrpdclient API change: application need not to know the mrpd socket port number
https://github.com/jbmaillet/Open-AVB/commit/1201e3b82907254dcf4d4e4a63368008e3d0835e

mrpdclient: API change, un-globify the socket connection to mrpd
https://github.com/jbmaillet/Open-AVB/commit/3be37c4a5390312c3f1ede00d0dcea3501491630

(...but I can't label it. Would someone p@lease add the "enhancement" + "MRP" labels?)

I have labelled it.