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.
6c82e2af92