Decode the 3GPP MSISDN AVP using the E.164 routines.

Rename dissect_e164_utf8_number() to dissect_e164_msisdn() and give it an
'encoding' argument.

Change-Id: I49cf5d2b24b44a0e69427ceae331f378024391c5
Reviewed-on: https://code.wireshark.org/review/3796
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>