swapoff: Remove only late devices with -aL.

[jilles]

Summary

Currently, '/etc/rc.d/swaplate stop' removes all swap devices. This can be
very slow and may not even be possible if there is a lot of swap space in
use. However, removing swap devices is only needed for late swap devices
that may depend on daemons that subsequent shutdown steps stop. Normal swap
devices such as hard disk partitions will remain available throughout the
shutdown process and need not be removed.

In swapoff, interpret -aL to remove late swap devices only, and use this in
etc/rc.d/swaplate. The meaning of -aL in swapon remains unchanged (add all
swap devices, both normal and late).

PR: 187081

Test Plan

Manual testing only: verified that a swap file was or was not removed after
'shutdown now'.

[wblock]

Man page changes look okay. Given the choice, I'd change all of them from "will be" to "are", but that's out of the scope of this change.

[ngie]

Thank you for fixing up the documentation for swapon(8). The change looks good -- thank you!