Make all uthread ops interrupt safe by default.

Previously the only interrupts we had were the alarms used by preemptive
schedulers.  This meant it didn't make sense to incurr the overhead of
enabling/disabling uthread interrupts on every yield/init, etc. call for
schedulers that didn't need it. With the change a few weeks ago to leverage
the interrupt mechanism for syscall interpositioning, it is now necessary to
handle all of this logic internal to the parlib library.