s3:connections: add a CNUM_OFFSET for mapping between cnum and the bitmap index

s3:connections: add a CNUM_OFFSET for mapping between cnum and the bitmap index

This moves the start of the range of valid cnum values up from 0 to CNUM_OFFSET
(currently 1), so that in a later step we can use 0 as invalid cnum value
instead of the current -1. This will allow us to change the type of cnum to
uint32_t from a mix of int and unsigned.

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>