MEDIUM: mworker/cli: keep the connection of the FD that ask for a reload

MEDIUM: mworker/cli: keep the connection of the FD that ask for a reload

When using the "reload" command over the master CLI, all connections to
the master CLI were cut, this was unfortunate because it could have been
used to implement a synchronous reload command.

This patch implements an architecture to keep the connection alive after
the reload.

The master CLI is now equipped with a listener which uses a socketpair,
the 2 FDs of this socketpair are stored in the mworker_proc of the
master, which the master keeps via the environment variable.

ipc_fd[1] is used as a listener for the master CLI. During the "reload"
command, the CLI will send the FD of the current session over ipc_fd[0],
then the reload is achieved, so the master won't handle the recv of the
FD. Once reloaded, ipc_fd[1] receives the FD of the session, so the
connection is preserved. Of course it is a new context, so everything
like the "prompt mode" are lost.

Only the FD which performs the reload is kept.