many: add ephemeral-mount-namespace feature

[zyga]

When the feature is enabled no mount namespace persistence is attempted. This is done in response to problematic behavior of ongoing updates, even under a frozen control group.

This change is disruptive and does not account for tests that currently observe this behavior.

For context: this is one of the two follow-ups identified in https://github.com/canonical/snapd/pull/14714

[zyga]

To test this you have to enable the feature:

sudo snap set core experimental.ephemeral-mount-namespace=true

EDIT: I've made that the default behavior now, to see how tests fare. You can still set it to false explicitly, to disable the feature.

[zyga]

I'm closing this as we do not expect to further make progress towards making mount namespaces ephemeral.