Add the PPID for 3GPP Pua. Move the (unregistered) M2TP PPID into that dissector.

Add the PPID for 3GPP Pua.  Move the (unregistered) M2TP PPID into that dissector.

Also put a note in sctpppids.h saying that only IANA-registered PPIDs should go
in that file.  Inspired by the rejection of
https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=4332

Change-Id: I763aad7d1b69e9d36c798061473438ce3cb66ca1
Reviewed-on: https://code.wireshark.org/review/434
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>
Tested-by: Anders Broman <a.broman58@gmail.com>