Add SourceDateEpochMode=override|clamp

[DaanDeMeyer]

In some use cases it's still useful to be able to override every timestamp so we can add a setting to allow configuring whether we clamp or override and default to clamp.

[keszybz]

Can you describe the use cases more? This might help in deciding how it's implemented and if it should ever be the default.

[DaanDeMeyer]

@keszybz @poettering mentioned he wants to be able to set the timestamp of everything in /usr to zero like ostree does. I figure it doesn't hurt to support that. I don't think it should be the default though.