rumpkern_demo: Work even though master socket is not fd 0

The WOPR demo was predicated on the master socket being fd 0.  But of
course it isn't because the rump environment now correctly starts with
some fds 0-2.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
rumpkern_demo: Return from readconn when read fails

After closing the fd, we should not continue with the rest of the
function.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
rumpkern_demo: Do not constantly try to close closed fds

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Config.mk: Make it possible to override XEN_HEADERS

This makes it possible to build against a checked out and built but
not installed xen tree, etc.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>