EAGAIN in "read jobs pipe"

[Ken Takusagawa]

For reasons I haven't completely triaged yet, I've been having my
parallel makes die with
"read jobs pipe: Resource temporarily unavailable.  Stop."

I hacked the function new_job in job.c such that if errno==EAGAIN,
then we sleep(1) and loop again, and my problem went away.
(Essentially adding EAGAIN to EBADF and EINTR as the ignored signals.)

--ken