BUG/MINOR: mworker/cli: reload fail with inherited FD

BUG/MINOR: mworker/cli: reload fail with inherited FD

When using the master CLI with 'fd@', during a reload, the master CLI
proxy is stopped. Unfortunately if this is an inherited FD it is closed
too, and the master CLI won't be able to bind again during the
re-execution. It lead the master to fallback in waitpid mode.

This patch forbids the inherited FDs in the master's listeners to be
closed during a proxy_stop().

This patch is mandatory to use the -W option in VTest versions that contain the
-mcli feature.
(https://github.com/vtest/VTest/commit/86e65f1024453b1074d239a88330b5150d3e44bb)

Should be backported as far as 1.9.