libnftables: Support buffering output and error

libnftables: Support buffering output and error

When integrating libnftables into Python code using ctypes module,
having to use a FILE pointer for output becomes a show-stopper.
Therefore make Python hackers' lives (a little) less painful by
providing convenience functions to setup buffering output and error
streams using fopencookie() and retrieving the buffers.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>