wireshark: use type-specific constructors for ProtoFields

This is apparently the more commonly used API. The generic constructor broke
compatibility between Wireshark releases 1.6 and 1.10.
wireshark: basic support for of_bitmap_128_t
Merge into master from pull request #85:
wireshark: fix compatibility with Wireshark 1.10 (https://github.com/floodlight/loxigen/pull/85)