TODO: bridge mode, finally?
TODO: gcm mode?
TODO: replace transport bits by transport endpoint structs?
TODO: http://incog-izick.blogspot.de/2011/08/using-openssl-aes-gcm.html
TODO: http://stackoverflow.com/questions/12153009/openssl-c-example-of-aes-gcm-using-evp-interfaces

TODO: verify
TODO: make sense of overhead calculation
TODO: if-up &c should not be scripts?
TODO: ipv6
TODO: gvpectrl should not use default privatekey,. or maybe document it better
TODO: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=828336