Feature request: Direct UID/GID use for -U

[malvidin]

Please add an option to directly use a user ID and group ID with the -U/--relinquish-privileges option.

With the following commands:

# tcpflow -w output_file_1 -U 900:990
# tcpflow -w output_file_2 -U 901
# tcpflow -w output_file_3 -U :991

I would expect the output files to have permissions like this.

# ls -la
-rw-r-----. 1 900  990  1000 Dec 19 10:20 output_file_1
-rw-r-----. 1 901  root 1000 Dec 19 10:20 output_file_2
-rw-r-----. 1 root 991  1000 Dec 19 10:20 output_file_3

[simsong]

Great idea. Can you submit a pull request with the modification?