startup: Don't mount /var as tmpfs

We create the /var and subdirectories in the book, there's no need to
make them tmpfs in startupscripts.
shutdown: Match echo line to actual rtc write
shutdown: Only swapoff if fstab exists

Or else swapoff complains about lack of fstab and apparently can't find
where swap is.
startup: Don't mount tmpfs on /dev

That's what DEVTMPFS is for and it's done automatically.
startup: Move /var/tmp symlink later

It cannot work if root is ro until after root is remounted rw.
startup/shutdown: Use /dev/rtc0

The first RTC will be number 0.
startup: Mount /dev/shm
startup: Fix mounting of /dev/pts
startup: Delete /var/run/*

As per FHS, this directory should be cleared at startup.