MINOR: cli: implement basic mechanism for uninterruptible jobs

MINOR: cli: implement basic mechanism for uninterruptible jobs

Interactive CLI commands will be interrupted when the CLI input
is disconnected. This is a safety measure to ensure a long running
command won't consume unnecessarily a CLI connection slot without any
means of shutting it down.

A disconnection is detected by the closure of the remote write side as
this is the only way to detect it when there is nothing to send.
However, this causes issues with utilities such as socat running with a
timeout which perform a half-close on their write channel when the
command has been sent. For example, it is currently impossible to
execute "wait" with socat, unless the special option "ignoreeof" is set.

Introduce a new concept of CLI uninterruptible jobs. This mode of
execution will only be available to long-running commands with a
dedicated argument. Under this mode of execution, the execution will
continue despite the remote write channel being already closed.

A limit of concurrent uninterruptible jobs is set to 3 to ensure they
won't exhaust the default CLI maxconn of 10.