Frame numbers are unsigned, and they start at 1; 0 is what's used for

Frame numbers are unsigned, and they start at 1; 0 is what's used for
"unknown" for frame numbers.  Note that in epan/frame_data.h, and make
the frame number in experts unsigned, and use 0 for "unknown", and
display it as an unsigned number - and, if it's 0, don't display it at
all.

Fix the signature of "expert_dlg_draw()" to match what a tap's draw
routine's signature is expected to be.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@15760 f5534014-38df-0310-8fa8-9805f1628bb7