add gsm48_pdisc_msgtype_name()

Composing the message type string requires knowing the protocol discriminator.
To ease printing the message type, add this function to switch between the
defined value_string[]s depending on pdisc.

Also publish the message type value_string[]s -- without inline functions to
access them because it is anyway more convenient to use
gsm48_pdisc_msgtype_name() instead.

Since gsm48_pdisc_msgtype_name() is nontrivial, do not add as inline function
-- in case the message type is not known, it needs a static string buffer.

Change-Id: I0fca8e95ed5c2148b1a7440eff3fc9c7583898df
3 files changed