Since rump rootfs now supports creating directories, mount ukfs
mounts at the requested mountpoint instead of as the rump kernel
rootfs.  While doing so, generally convert everything to use system
calls instead of handcrafter rump interfaces (e.g. rump_sys_mount()
instead of rump_mnt_mount()).  This avoids a lot of unnecessary
fuss.

Change UKFS_DEFAULTMP from / to /ukfs and bump ukfs version.

(pooka)