vfs_fake_dfq: add vfs module

vfs_fake_dfq: add vfs module

Add a vfs module "vfs_fake_dfq" for mocking disk-free
and user/group quota functions.

In this commit there are quota considerations in
disk_free calculations, based on the mocking of quota.

Those considerations will later be removed once we
refactor the server code to weigh the disk-free and quota in
the smb layer and not in individual vfs implementations.

Signed-off-by: Uri Simchoni <uri@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>