Wait for POLLPRI on /proc or /sys files.

Wait for POLLPRI on /proc or /sys files.

From 2.6.30, /proc/mounts and various /sys files will
probably always returns 'readable' to select, so we will need
to wait on POLLPRI to get the 'new data is available' signal.

When using select, this corresponds to an 'exception', so
adjust calls to select accordingly.
In one case we sometimes wait on a socket and sometime on
/proc/mounts, so we need to test which.

Signed-off-by: NeilBrown <neilb@suse.de>