Rename proto_construct_dfilter_string() to

Rename proto_construct_dfilter_string() to
proto_construct_match_selected_string() to indicate what it does - and
have it return a Boolean indication of whether the string could be
built, returning the string through a pointer, and, if that pointer is
null, have it just return the Boolean and not construct the string.

Get rid of proto_can_match_selected() -
proto_construct_match_selected_string() can be used for that, which
means we have only one piece of code that knows whether a "match
selected" string can be constructed or not.

Have proto_construct_match_selected_string() support matching
zero-length FT_NONE (and FT_PCRE, but that shouldn't happen) fields even
if there's no epan_dissect_t, as such a match just checks whether the
field is present.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@19967 f5534014-38df-0310-8fa8-9805f1628bb7