The value returned by the open_* routines, called 'a handle', is
nothing but an index into the static table of library contexts (one per
open device). The possibility that a handle is zero can be bothersome
for clients that assume it has some kind of file descriptor semantics.
So we make it start by 1 instead of 0 to avoid special, different
treatment between MagentaSYS attns and the USB type of attn.

Reported-by: Dimitris Lampridis <dimitrios.lampridis AT cern.ch>

This was all REVERSED in the original library, causing endless
confusion. Now, explicit ATT_MINUS_20dB labels are used, so no confusion
is possible.