interrupts masked, we can't use the old mechanism of delivering
signals immediately when they arrive. Signals must instead be
delivered when the asyncs run *after* the end of the critical
section in scm_internal_select. But this also means after context
switch so that the signal will be delivered to a different thread.
To avoid this, I have changed the protocol of
coop_wait_for_runnable_thread and friends so that they are allowed
to return the original thread. So, if a signal arrives during
scm_internal_select, we won't any longer be forced do a context
switch, but can remain in the same thread and deliver the signal
to it.
* iselect.c: Small fixes.
