msg/async: return right away in NetHandler::set_priority() if not supported

msg/async: return right away in NetHandler::set_priority() if not supported

* SO_PRIORITY is linux specific, so no need to check __linux__
* early return if priority is less than 0 (maybe we should also return if
  it's higher than 6?), the less indent.
* store errno if fails to set SO_PRIORITY before printing log messages.
* guard the whole function with '#ifdef SO_PRIORITY' so on platforms
  where this option is not supported, this function will be a no-op.

Signed-off-by: Kefu Chai <kchai@redhat.com>