Use a more deadlock-free `wrReqs` main loop.  As part of this, also add limited blocking (configurable tmout) selects for more CPU friendliness.
Use a more deadlock-free `wrReqs` main loop.  As part of this, also add
limited blocking (configurable tmout) selects for more CPU friendliness.

(Prior version could fill all kid input buffers and block in the select
even though each individual fd was non-blocking, but then being blocked
will never read/free room by reading replies leaving kids blocked.)

Performance implications (as assessed by soon to be added `ppBench`) are
positive (~2X lower overhead on 4-core Skylake) or neutral (within noise
of measurement error on 16-core AMD/ancient Intel).

c-blake/cligen