Relinquish privileges when running tcpdump

In some scenarios, when tcpdump is running as root, after opening the capture device or input savefile, but before opening any savefiles for output, "-Z user/--relinquish-privileges=user" option can be used to switch to another user and drop privileges. E.g.:

# sudo tcpdump -Z nan
dropped privs to nan
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
......