proto-shell: allow running protocols without proto_task.

Adds a new config parameter "no-proto-task" for noting that no
proto_task will be running for this protocol type.  This is required
since then change in commit "d0dcf74 proto-shell: retry setup if the
proto handler script quits without changing the state or starting a
process".

The change is mainly for protocols like xl2tpd in which control commands
are sent to another daemon xl2tpd to start L2TP negotiation and pppd
process who is not under netifd's control as proto_task as is the case
in other ppp related protocols like pppoe, pptp, etc.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>