MINOR: threads: introduce a minimalistic notion of thread-group

MINOR: threads: introduce a minimalistic notion of thread-group

This creates a struct tgroup_info which knows the thread ID of the first
thread in a group, and the number of threads in it. For now there's only
one thread group supported in the configuration, but it may be forced to
other values for development purposes by defining MAX_TGROUPS, and it's
enabled even when threads are disabled and will need to remain accessible
during boot to keep a simple enough internal API.

For the purpose of easing the configurations which do not specify a thread
group, we're starting group numbering at 1 so that thread group 0 can be
"undefined" (i.e. for "bind" lines or when binding tasks).

The goal will be to later move there some global items that must be
made per-group.