More Sysdig / system event support.

More Sysdig / system event support.

Add REC_TYPE_SYSCALL to wiretap and use it for Sysdig events. Call the
Sysdig event dissector from the frame dissector. Create a "syscall"
protocol for system calls, but add "frame" items to it for now.

Add the ability to write Sysdig events. This lets us merge packet
capture and syscall capture files.

Change-Id: I12774ec69c89d8e329b6130c67f29aade4e3d778
Reviewed-on: https://code.wireshark.org/review/15078
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>