Install FreeBSD.app: Allow upgrades and/or clean installs preserving /home

**Clean install (current behavior):** On the selected disk, delete the partition table (using `/sbin/gpart destroy -F "${INSTALLER_DEVICE}"`, and then create new partitions using `bsdinstall zfsboot`. As a result, the root filesystem will be mounted at `FSMNT="/mnt"` and the installation proceeds.

**Clean install preserving `/home`**: Like clean install, but do not delete the partition table, and leave `/home` alone but do delete all other partitions.

**Upgrade:** Check for existing disks that contain a helloSystem installation, delete all files that were previously installed there by the helloSystem installer, and then install the new helloSystem version there. (Actually, optimize for speed by only deleting the files that won't be overwritten.)