nixos/zfs: Keep zero-sized auto snapshots by default

Otherwise, in certain cases, snapshots of infrequently-modified
filesystems can be kept for a much longer time than the user would
normally expect, and cause a large amount of extra disk space to be
consumed.

Also added flag to snapshot filesystems in parallel by default.

I've also added a configuration option for zfs-auto-snapshot flags, so
that the user can override them.

For example, the user may want to append --utc to the list of default
options, so that the snapshot names don't cause name conflicts or
apparent time reversals due to daylight savings or timezone changes.