fix-encoding-args.pl: Add a few additional fixups;

Specifically: A number of cases of the use of the encoding arg
              "ENC_ASCII | ENC_NA" for FT_BYTES FIELDTYPEs seem to
              have crept into Wireshark source.

Also: Add a ToDo comment as to code rework needed to be able to
      better *validate* encoding args.

Change-Id: I9d4b10b869fe2aef3f20bcd2d7ad6531375aba87
Reviewed-on: https://code.wireshark.org/review/4183
Reviewed-by: Bill Meier <wmeier@newsguy.com>