installer: Utilize dumpon(8) and swapoff(8) to turn off dump and swap.

installer: Utilize dumpon(8) and swapoff(8) to turn off dump and swap.

Previously, the installer would block further installation attempts
once a swap was mounted. This was very inconvenient, e.g. when trying
to do multiple installs in one session.

It also never issued a 'dumpon off' causing dumping to be enabled
until rebooting. This caused issues upon reboot when a removable disk
(eSATA, for example) was unplugged before the reboot.

Now that we have swapoff(8), fix all these issues by turning off
dumping and swapping once the installation is completed (after
unmounting). Also, do this in a number of other places (where we
previously warned) to make sure it all gets deactivated again should
the user have cancelled a previous installation.