proto: Decode negative values in a bitmask field

proto: Decode negative values in a bitmask field

Allows to use negative values (FT_INT) in a bitmask field.
If a field type greater or equal (FT_(U)INT40) is used then also
a 64 bit value string and 64 bit format function must be used.

Change-Id: Ib86d45bee73a71a784068ab717d35810c6f48017
Reviewed-on: https://code.wireshark.org/review/18601
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Guy Harris <guy@alum.mit.edu>