Skip privilege dropping when using -Z root on --with-user builds

[martinwilli]

Distributions which started building --with-user to switch to an unpriviliged user claim that the old behavior of running under root can be restored by passing "-Z root" on the command line. However, doing so is different from not using --with-user, as tcpdump still drops privileges and sets supplementary user groups.

In Linux containers using user namespaces with an in-container root user mapped to an unprivileged external user, calling setgroups() is usually denied, as it would allow that unprivileged user to leave groups (see user_namespaces(7) for details). Passing "-Z root" on a --with-user build still goes through initgroups() and therefore setgroups(), which will fail in such a container environment. This makes tcpdump builds using --with-user effectively unusable in such containers.

Adjust the "-Z root" fallback to skip any privilege dropping and supplementary group setup, making it identical to builds not using --with-user.