send_cmd: throttle `sleeping on sendmsg' messages

send_cmd: throttle `sleeping on sendmsg' messages

Emitting a message every 100ms was too much for busy machines.
Throttle it to 1.6s to avoid flooding user terminals by emitting
every 16 sleeps.  A power-of-two (16) was chosen for its
optimization potential via bitwise AND.  Since perl(1) doesn't
do this optimization, we open code the bitwise AND.  I don't
assume an optimizing compiler for Inline::C, either, since I
find working in C far more enjoyable with optimizations off.