r5063: Shamelessly steal the Samba4 logic (and some code :-) for directory

r5063: Shamelessly steal the Samba4 logic (and some code :-) for directory
evaluation. This stops us from reading the entire directory into
memory at one go, and allows partial reads. It also keeps almost
the same interface to the OpenDir/ReadDir etc. code (sorry James :-).
Next I will optimise the findfirst with exact match code. This speeds
up our interactive response for large directories, but not when a
missing (ie. negative) findfirst is done.
Jeremy