vfs: Add pivot_root() system call

This adds a simple pivot_root() system call that works on mountpoints
and simply removes 'put_old' from mount list so that VFS doesn't know
about it and adds renames 'new_root' ->m_path to '/'.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
ramfs: unmount

Needed by pivot_root() to unmount the initial rootfs.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
ZFS root filesystem

Use the new pivot_root() functionality to switch to ZFS root filesystem
once OSv is up and running.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
scripts/test.py: Show running test case name

Show running test case name.  Makes debugging test failures less
painful...

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>