BUG/MEDIUM: thread: fix extraneous shift in the thread_set parser

BUG/MEDIUM: thread: fix extraneous shift in the thread_set parser

Aurélien reported a bug making a statement such as "thread 2-2" fail for
a config made of exactly 2 threads. What happens is that the parser for
the "thread" keyword scans a range of thread numbers from either 1..64
or 0,-1,-2 for special values, and presets the bit masks accordingly in
the thread set, except that due to the 1..64 range, the shift length must
be reduced by one. Not doing this causes empty masks for single-bit values
that are exactly equal to the number of threads in the group and fails to
properly parse.

No backport is needed as this was introduced in 2.8-dev3 by commit
bef43dfa6 ("MINOR: thread: add a simple thread_set API").