Add new parameter, "min receivefile size" (by default set

Add new parameter, "min receivefile size" (by default set
to zero). If non-zero, writeX calls greater than this
value will be left in the socket buffer for later handling
with recvfile (or userspace equivalent). Definition of
recvfile for your system is left as an exercise for
the reader (I'm working on getting splice working :-).
Jeremy.
(This used to be commit 11c03b75ddbcb6e36b231bb40a1773d1c550621c)