dnsdist: Keep retained capabilities even when switching user/group

dnsdist: Keep retained capabilities even when switching user/group

On Linux, we support retaining some capabilities if we are running
as root (eeew) or as an unprivileged user with ambiant capabilities,
but we did not yet support keeping these if we were started as root
but then switched to a different user ID and/or group ID.
This commit uses `PR_SET_KEEPCAPS`, when available, to do just that,
to be able to retain the capabilities we need without running as a
fully privileged users even when we cannot easily use ambiant
capabilities.