FAQ: Extend the forgot-password question to include kbdmap and ZFS

When going into single user mode to change the root password, some users
may want to change to their preferred keyboard layout before entering a
new password. Add a sentence about how to do that.

When remounting all the remaining filesystems, there are different mount
instructions for UFS and ZFS. Mention how to do that for each of them.

PR: 294971