beam: Add simple framing system for UnixConn

This is needed for Send/Recieve to correctly handle borders between
the messages.

The framing uses a single 32bit uint32 length for each frame, of which
the high bit is used to indicate whether the message contains a file
descriptor or not. This is enough to separate out each message sent
and to decide to which message each file descriptors belongs, even
though multiple Sends may be coalesced into a single read, and/or one
Send can be split into multiple writes.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)