proto-shell: Support teardown on layer 3 link loss

proto-shell: Support teardown on layer 3 link loss

Commit c6858766 added interface teardown support on layer 3 device link loss
mainly for shell protocols who have no proto task like xl2tp. However for
shell protocols having a proto task it is not always the correct action to
teardown the interface; as an example the PPP daemon can be put into
persist state trying to re-establish the link via a hold-off mechanism
if layer 3 link loss is detected.
Therefore shell handlers can enable via TEARDOWN_ON_L3_LINK_DOWN a proto
flag which will teardown the interface when layer 3 link loss is detected

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>