Removed mountNow as a top-level property, as I don't think it makes
sense for anything except for mounted to use it.
db45x's patch turns out to have introduced a bug in mounted's use of
"mountNow src". That made mountNow check if the device was a mount
point, which it isn't. The fix would have been to use "mountNow mnt",
but my inlining of mountnow just basically reverted the part of the
patch that introduced the bug.
swapOn does not involve the fstab so moved to the Mount module.
(Also noticed that Mount.mounted is a kind of weird property, given that
it fails the next time ran. It's only used internally by some chroot
properties, so I left it as-is, but added a comment. It might make sense
to make Mount.mounted check like mountNow does if the thing is already