mgw_nat: Add ability to translate multiple signalling links

The mgw_nat_sup supervisor now starts one mgw_nat_usr process for each
MSC-STP link defined in the configuration file.  However, the actual
translation/mangling/masquerading configuration as well as runtime state
like allocated SCCP mappings is global/shared between all signalling
links.

Furthermore, a new mgw_nat_adm process is introduced to ensure config
file reloading (formerly handled by the single mgw_nat_usr) does not
have to run in the supervisor (and risk crashing it).
5 files changed