batctl: Add support for hardif settings

batctl: Add support for hardif settings

batctl currently supports settings which are either mesh interface or vlan
specific. But B.A.T.M.A.N. V introduced two additional settings which are
hard (slave) interface specific.

To support these, an additional command prefix called hardif is implemented
for some sysfs commands:

  $ batctl -m bat0 hardif eth0 ..

The usable commands with that are:

* elp_interval
* throughput_override

Signed-off-by: Sven Eckelmann <sven@narfation.org>