OML: Return attributes in ACK/NACK messages
OML: Return attributes in ACK/NACK messages

As per 3GPP TS 12.21 Section 8.2 "ACK messages shall return all the
attributes in the original message". OsmoBTS fails to do so but simply
sends no attributes at all in the ACK.

TS 12.21 is a bit vague whether or not the attributes shall also be
achoed in the NACK.  Let's do it and append the CAUSE in this case.

Closes: OS#3788
Change-Id: Ifb305fe75f8305bb04872f26492b8b1bb8c27f49

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts