MEDIUM: poller: disable thread-groups for poll() and select()

MEDIUM: poller: disable thread-groups for poll() and select()

These old legacy pollers are not designed for this. They're still
using a shared list of events for all threads, this will not scale at
all, so there's no point in enabling thread-groups there. Modern
systems have epoll, kqueue or event ports and do not need these ones.

We arrange for failing at boot time, only when thread-groups > 1 so
that existing setups will remain unaffected.

If there's a compelling reason for supporting thread groups with these
pollers in the future, the rework should not be too hard, it would just
consume a lot of memory to have an fd_evts[] array per thread, but that
is doable.