mkdir and rmdir in .zfs/snapshot to create and destroy snapshots

[ilovezfs]

Not sure if this requires the daemon or if the kernel alone can figure this one out.

[lundman]

Just minor porting required in zfs_ctldir.c to have vnops for mkdir, rmdir. Solaris code is there, just needs to be applified.

Perfect for someone who would like to cut their teeth on some kernel development.