ssh: add support for netconf 1.1

Signed-off-by: Luka Perkov <luka.perkov@sartura.hr>
ubus: drop reverse ssh support

We are going to release a different tool to accomplish this feature.

Signed-off-by: Luka Perkov <luka.perkov@sartura.hr>
ssh: switch to the new ssh subsystem

Signed-off-by: Luka Perkov <luka.perkov@sartura.hr>