Don't retry fork() in a loop.

Don't retry fork() in a loop.

If fork() fails, then report a fatal error.  Don't spin the cpu
retrying fork() until it succeeds.  It can fail when we reach a limit
on the number of processes.  Spinning on the cpu would slow down other
processes when we want them to exit.  This would get bad if we had a
parallel build with multiple ack processes spinning.