unixsock plugin: Open two different IO stream handles for reading and writing.

unixsock plugin: Open two different IO stream handles for reading and writing.

Full-duplex standard IO streams are not really supported on sockets.
Mixing input and output functions involves calls to lseek(2) which is
not supported on sockets and thus causes the IO operations to fail.
Opening two IO streams solves the problem.

This is a backport of 43df21461d523023951746ef669f1bb95f61366d.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>