Add a routine to dissect an UTF8-encoded IMSI (as you'd find in Diameter) and use it to decode the User-Name AVP of S6a/S6d messages.

Also clean up some white space/formatting.

Change-Id: Idf2d4cad2af1826b24de7d59407ffeb36100c19b
Reviewed-on: https://code.wireshark.org/review/3742
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>