Bug 36056: Clarify subpermissions AND behavior

Bug 36056: Clarify subpermissions AND behavior

Working on bug 31791, I found myself wondering if our current recursive
code in C4::Auth::haspermission() would allow checking AND on
subpermissions.

As it is not documented in the POD or tested, I decided to write some
unit tests for it.

It turned out it was well supported, so I decided to submit the tests,
and a small tweak in the POD to reflect that.

To test:
1. Apply this patch
2. Run:
   $ ktd --shell
  k$ prove t/db_dependent/Auth/haspermission.t
=> SUCCESS: Tests pass! The code supports AND on subpermissions!
3. Sign off :-D

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>