Rather than specifying None in needed_locks every time, with a nice
comment saying to read what we mean rather than what we write, and that
None actually means All, in our magic world, we'll hide this secret
under the ALL_SET constant in the locking module, which has value, you
guessed it, None. After that we'll substitute all usage in cmdlib.

Some comments and examples have been fixed as well.

Reviewed-by: iustinp