Reduce code duplication by moving parsing of "QUBESRPC" magic command to
one place.
Call qubes-rpc-multiplexer directly with execve(), to avoid string
expansions in its parameters.
Instead of pipes, stdin/out/err are created as sockets. This allows
qrexec-agent/daemon to decide to use some of them bidirectional. This is
up to qrexec-agent/daemon, such socket can still be used as
unidirectional channel.
The main reason for this feature is to use USBIP over qrexec, which
require single socket.
