proto-shell: add checkup timeout to restart interface.

This is mainly for protocols with no_proto_task set.  L2TP with xl2tpd
is such a case and the issue this commit tries to address is that xl2tpd
could fail redialing the connection (segfault or abort) without the
notice of netifd causing the concerned interface being left down.

This patch solves it by allowing users to configure an timeout value
instructing netifd to check if the interface is in up state after its
last attempt to setup it and try again if that is not the case.

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