dropbear: enable configurable port forwarding options

dropbear: enable configurable port forwarding options

Currently its only possible to disable port forwarding only for specific
keys, via the OpenSSH-style restriction in `authorized_keys` file.

In some use cases it might be feasible to disable such features globally
on service level, so lets add new LocalPortForward and RemotePortForward
config knobs.

Signed-off-by: Petr Štetiar <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/21071
Signed-off-by: Christian Marangi <[email protected]>