Don't change effective uid when we already dropped privileges

Don't change effective uid when we already dropped privileges

When running on Linux and system capabilities are available, named will
drop the extra capabilities before loading the configuration.  This led
to spurious warnings from `seteuid()` because named already dropped
CAP_SETUID and CAP_GETUID capabilities.

The fix removes setting the effective uid/gid when capabilities are
available, and adds a check that we are running under the user we were
requested to run.

(cherry picked from commit 6c82e2af92c741b52585e67054a67862211875b0)