To better reflect what the field is: field 'module' is the dictionary
module that's calling diameter_gen to decode a list of AVP, while field
'app_dictionary' is the dictionary module defining the message being
decoded.

{record_decode, map} is a bit too quirky.

To control whether or not messages and grouped AVPs are decoded to
records, in #diameter_packet.msg and #diameter_avp.value respectively.
The decode became unnecessary for diameter's needs in parent commit,
which decoupled it from the checking of AVP arities.

The previous commit ensures that only one will be reported in an answer
message when diameter itself sets Result-Code/Failed-AVP.

The order of errors in #diameter_packet.errors is that in which they're
detected, not the reverse as previously.