var/loop/select: stop iteration generator while waiting for reply
var/loop/select: stop iteration generator while waiting for reply

This makes 'LOOP select' much better behaved: we avoid filling up
the interprocess buffer while still remaining responsive if a user
gives many replies in quick succession.

lib/modernish/mdl/var/loop/select.mm:
- _loop_select_iterate():
  - Make the iteration generator determine its PID and stop itself
    with SIGSTOP after writing each iteration command.
  - Don't keep re-quoting the arguments on every iteration; instead
    use 'shellquoteparams' to quote them in-place once, and use
    'put' to separate them with spaces in the output.
- _loop_select_getReply():
  - Take the PID of the iteration generator from the 2nd argument.
  - Send SIGCONT to the iteration generator after getting a reply,
    so it writes the next iteration while the loop body runs.

modernish/modernish