Add support for the unless-stopped restart policy

We initially believed that implementing this required support for
restarting containers after reboot, but this is not the case.
The unless-stopped restart policy acts identically to the always
restart policy except in cases related to reboot (which we do not
support yet), but it does not require that support for us to
implement it.

Changes themselves are quite simple, we need a new restart policy
constant, we need to remove existing checks that block creation
of containers when unless-stopped was used, and we need to update
the manpages.

Fixes #6508

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

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