VoIP calls: Clean up tap registration and removal.

Start moving away from a global voip_calls_tapinfo_t. Pass the
voip_calls_tapinfo_t struct address to each registration and removal
routine.

All of the tap registration and removal routines are called at the same
time. Replace the individual registration tracking booleans with a
central one.

Calling exit() from the GUI is rude. Don't do that.

Change-Id: I9535002e2905a45a8cfc9643590b5d2d360ffc24
Reviewed-on: https://code.wireshark.org/review/5318
Reviewed-by: Gerald Combs <gerald@wireshark.org>