There is no reason to use a single message for nonoverlapping requests
and replies combined, and in fact splitting them out allows reuse of
messages and avoids various problems with field layouts. Since the
upcoming socketpair(2) system call will be using the same reply as
pipe2(2), split up the single message used for the latter. In order
to keep the used parts of messages at the front, start a transitional
phase to move the pipe(2) flags field to the front of its request.
Change-Id: If3f1c3d348ec7e27b7f5b7147ce1b9ef490dfab9
c33d6ef392