utils: Prevent critical section clients to talk to each other.

* src/cuirass/utils.scm (make-critical-section): Put the modified message to
the REPLY channel that was part of the initial message.
(call-with-critical-section): Create a REPLY channel, add it to the sent
message, get the modified message from that channel.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>