devfs rewrite:
- implement VFS_OPEN_NODE, VFS_DEVICE, VFS_SYNC, VFS_CLOSE
- connect to underlying device during VFS_LOOKUP (if L_OPEN flag is used) and VFS_OPEN_NODE
- disconnect from underlying device during VFS_CLOSE
- forward VFS_READ, VFS_WRITE and VFS_SYNC to the underlying device
- VFS_DEVICE: return underlying device handle