t/httpd-unix: eliminate some busy waits

t/httpd-unix: eliminate some busy waits

A small step towards making our test suite as sleep-less as
possible.  We can use FIFOs to coordinate processes in a few
places, while other spots can take advantage of disabling
FD_CLOEXEC to further eliminate back-and-forth traffic between
processes.

This speeds up t/httpd-unix.t by ~20 ms on my system.