MEDIUM: listener: deprecate "process" in favor of "thread" on bind lines

MEDIUM: listener: deprecate "process" in favor of "thread" on bind lines

The "process" directive on "bind" lines becomes quite confusing considering
that the only allowed value is 1 for the process, and that threads are
optional and come after the mandatory "1/".

Let's introduce a new "thread" directive to directly configure thread
numbers, and mark "process" as deprecated. Now "process" will emit a
warning and will suggest how to be replaced with "thread" instead.
The doc was updated accordingly (mostly a copy-paste of the previous
description which was already up to date).

This is marked as MEDIUM as it will impact users having "zero-warning"
and "process" specified.