babeld: Backward compatibility with old config files

babeld: Backward compatibility with old config files

This allows to use the old syntax "config interface foo" for specifying an
interface.  The new syntax ("option ifname foo") takes precedence.

Also, old option names for the "general" section are translated
accordingly.  Exceptions are:

- 'hello_interval' and 'wired_hello_interval', which are now interface-specific;
- 'conf_file', since /etc/babeld.conf is now always used;

which are simply ignored.

Beware, the syntax for the 'diversity' option is not backward compatible.