networking: extend API for better memory management

networking: extend API for better memory management

Networking backend implementations may need to allocate dynamic
resources that require an explicit free/release.
Since these cleanup are perfomed not very often, and only at specific
times, it makes sense to have the upper layer signal when it's the right
time to do so, by means of a new API call.

For this purpose two news APIs have been implemented:
- net_ctx_free() to release all backend specific resources. Expected to
  be called at application cleanup time;
- net_ctx_reset() to let backends release temporary resources (i.e.
  reset garbage collectors). To be invoked after routines that
  are expected to allocate memory (i.e. tun setup or shutdown).

In this patch related implementations for iproute2 and sitnl are also
provided.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20190816202654.19388-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18780.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>