Implement sendmsg to pass control messages

[rm5248]

I'm attempting to pass a file descriptor between two applications via dbus-java, but since this is not currently implemented correctly it seems as though I need to add support to jnr-unixsocket in order to properly pass the file descriptors between the processes. What's the best way to extend the classes in order to accomplish this task? It seems like this requires a completely new class, since the current implementation is only worried about writing messages using the write system call.

[rm5248]

This is actually already implemented as part of jnr-posix. Closing.