MEDIUM: config: restrict shards, not bind_conf to one group each

MEDIUM: config: restrict shards, not bind_conf to one group each

Now that we're using thread_sets there's no need to restrict an entire
bind_conf to 1 group, the real concern being the FD, we can move that
restriction to the shard only. This means that as long as we have enough
shards and that they're properly aligned on group boundaries (i.e. shards
are an integer divider of the number of threads), we can support "bind"
lines spanning more than one group.

The check is still performed for shards to span more than one group,
and an error is emitted when this happens. But at least now it becomes
possible to have this:

    global
       nbthread 256

    frontend foo
       bind :1111 shards 4
       bind :2222 shards by-thread