Drop privileges very early on.

TShark relies on dumpcap to capture packets, and TFShark doesn't even do
packet capturing (it dissects files, not network traffic), so neither of
them need, or should run with, special privileges.  If you *must* run
with special privileges in order to capture, grant those privileges to
dumpcap, which has a *lot* fewer lines of code than libwireshark and
TShark/TFShark.

Change-Id: I8f8fedead355ca163895e025df37240d2f232ba4
Reviewed-on: https://code.wireshark.org/review/2736
Reviewed-by: Guy Harris <guy@alum.mit.edu>