Add example scripts for the distributed GSM network:

esme_dgsm.py: connect to the SMPP port of OsmoMSC A and forward SMS to the SMPP
port of OsmoMSC B. The IP and port of OsmoMSC B is retrieved by the receiver's
MSISDN using osmo-mslookup-client.

contrib/dgsm/freeswitch_dialplan_dgsm.py: resolve the destination SIP servers
of calls with osmo-mslookup-client and bridge the calls accordingly.

For a detailed overview of the D-GSM and mslookup related files, please see the
elaborate comment at the top of mslookup.c (already added in an earlier patch).

Related: OS#4254
Related: OS#4255
Change-Id: I26e8dd8d9a08187fccb3e74ee91366bc24f6c608