MINOR: threads: add a new function to resolve config groups and masks

MINOR: threads: add a new function to resolve config groups and masks

In the configuration sometimes we'll omit a thread group number to designate
a global thread number range, and sometimes we'll mention the group and
designate IDs within that group. The operation is more complex than it
seems due to the need to check for ranges spanning between multiple groups
and determining groups from threads from bit masks and remapping bit masks
between local/global.

This patch adds a function to perform this operation, it takes a group and
mask on input and updates them on output. It's designed to be used by "bind"
lines but will likely be usable at other places if needed.

For situations where specified threads do not exist in the group, we have
the choice in the code between silently fixing the thread set or failing
with a message. For now the better option seems to return an error, but if
it turns out to be an issue we can easily change that in the future. Note
that it should only happen with "x/even" when group x only has one thread.