[99fs-lib] Create generic mount framework

* Utilises fstab.{early,late} files in /etc
* Provides fs_add_mount function to mount in pre-mount (early) and pre-pivot (late) phases
* Code to mount one is moved over from 95fstab-sys
* Uses pre-mount and pre-pivot hooks, so cannot be used to mount rootfs
* Also provides fs_mount_to_var utility function to parse /path:type:/dev,options style mountspecs