MINOR: cli: store active CLI sessions in a global list

MINOR: cli: store active CLI sessions in a global list

Implement a global list of the currently active CLI sessions. This is
sound as since only one CLI frontend exists per HAProxy instance (via
"stats socket" for a worker, or alternatively a master CLI via -S for a
master process).

Each session is appended in the CLI_ST_INIT state at the beginning of
cli_io_handler() and removed in cli_release_handler().

This will allow to implement new CLI commands which can interact with
other CLI sessions.