GSUP: Convert PDP-Type IE to PDP-Address IE

The previous PDP-Type IE should have been a PDP-Address from the
start, since having only PDP-Type with no address is only a specific
case (dynamic addressing).
This becomes clear by looking at other similar protocols like:
* MAP: APN-Configuration IE has servedPartyIP-IP{v4,v6}-Address IEs
* Diameter S6b, 3GPP TS 29.272 7.3.35 APN-Configuration contains
  Served-Party-IP-Address AVPs
* Diameter SWx, 3GPP TS 29.273 APN-Configuration.
* GTPv1C Ts 29.060 7.7.29 PDP Context containing PDP Address.

Since PDP-Type on its own really makes no sense, being it a special case
of PDP-Address, let's keep the IE by renaming it (keeping old name too
for API backward compat) and extend it to support lengths > 2 bytes.

Old implementation of libosmogsm gsup actually ignored lengths > 2
bytes, so we are safe acting against older implementations here, both
on the sending and receiving side on the wire.

Change-Id: I3e92368fff61694bcef6a48320595b59ae8f54ca
Related: OS#6091
Related: libosmocore.git Change-Id I775ff9c3be165d9f30d6ab55d03f99b6104eadd6
Related: osmo-gsm-manuals.git Change-Id I775ff9c3be165d9f30d6ab55d03f99b6104eadd6
---
M library/GSUP_Templates.ttcn
M library/GSUP_Types.ttcn
M sgsn/SGSN_Tests.ttcn
3 files changed, 91 insertions(+), 12 deletions(-)