Show BER identifiers and lengths in SNMP variable bindings.

If the user turns on the "show internal BER encapsulation tokens"
preference, show them the tokens inside SNMP variable bindings.

Change-Id: Ief9040f422cb214bbff8e4cfd45a2e05c7106480
Reviewed-on: https://code.wireshark.org/review/4105
Reviewed-by: Guy Harris <guy@alum.mit.edu>