darwin_thread: use correct value for count parameter to thread_policy_set()
signal_cpualarm: use gettimeofday() to measure proclet running time

The thread_info() call on Darwin was incurring the most overhead
on each proclet context switch. thread_info() is implemented as a
Mach system call and this is very slow. We were using the thread_info()
call to determine the total CPU time used by a process so we can measure
the amount of time used while it's running and know when to preempt.
gettimeofday() changes the semantics of cpu_alarm_reset() but it's not
necessarily incorrect and it's much faster than thread_info().
gettimeofday() is a unique system call on XNU (Darwin) and Linux because
instead of switching into kernel mode, it reads a memory page that is
shared and asynchronously updated by the kernel.