lib: introduce lldpctl_watch_callback2()

lib: introduce lldpctl_watch_callback2()

This is similar to `lldpctl_watch_callback()` (which is getting
deprecated), except the callback won't receive the current connection.
This prevents a user to use the connection which is unusable because
it is now dedicated to watch events.

Minor ABI dump due to new function, but everything is
backward-compatible, except you may now get an error if you use the
connection while watching (but this was already not supported).

Fix #380