Moved global memory alloction used in display filters (which was stored

Moved global memory alloction used in display filters (which was stored
in dfilter-grammar.y) to a new struct dfilter. Display filters now have
their own struct, rather than simply being GNode's. This allows multiple
display filters to exist at once, aiding John McDermott in his
work on colorization.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@480 f5534014-38df-0310-8fa8-9805f1628bb7