Wiretap: Added file_seek SEEK_END support.

This is needed for Lua File:seek("end").

Change-Id: I28fb23f2f29ca8083c77bf065db8816e039ae5a1
Reviewed-on: https://code.wireshark.org/review/4722
Reviewed-by: Michal Labedzki <michal.labedzki@tieto.com>
Tested-by: Michal Labedzki <michal.labedzki@tieto.com>
Petri-Dish: Michal Labedzki <michal.labedzki@tieto.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>