The batadv debugfs requires large memory blocks to write the text debug
tables. This is inefficient for large tables like the global translation
table or the originators table.

The memory requirement can be reduced by using netlink. It copies smaller
packets in a binary format to the userspace program. gluon-status-page-api
can therefore parse larger originator tables without causing an OOM on
systems which are tight on memory.

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

Should be redundant after 0bd0df6f

gluon-announced and gluon-announce are merged to gluon-respondd.

introduce function to recurse down to the lowest layer-2 interface
corresponding to a given interface.
also re-introduce some of the previously removed input validation plus
some more to protect against glob and path based exploits.