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

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

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.