libcontainer: configs: initial support for timens

[metux]

Allow explicit unsharing of time namespace.

Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net

[cyphar]

This isn't really sufficient, in order to really make use of the timens you need to configure the time offsets when the container first starts (and that would require switching nsenter to use clone3, as well as figure out a reasonable way to configure it -- which will probably require runtime-spec changes).

[kolyshkin]

@metux if you want to work on that, the first step is working on the proposal in opencontainers/runtime-spec#1062, which is still pending. Once merged, we can look into implementing timens.

Closing for now as much more changes are needed than adding a few constants (for more details, see #2345).