MEDIUM: cli: handle CLI level from the master CLI

MEDIUM: cli: handle CLI level from the master CLI

Handle the CLI level in the master CLI. In order to do this, the master
CLI stores the level in the stream. Each command are prefixed by a
"user" or "operator" command before they are forwarded to the target
CLI.

The level can be configured in the haproxy program arguments with the
level keyword: -S /tmp/sock,level,admin -S /tmp/sock2,level,user.