Refactor get_interface_metric to return metric and auto flag separately

Refactor get_interface_metric to return metric and auto flag separately

- Instead of returning metric = 0 when automatic metric is in use
  return the actual metric and flag automatic metric through a
  parameter. This makes the function reusable elsewhere.

- Ensure return value can be correctly cast to int and return -1 on
  error.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Simon Rozman <simon@rozman.si>
Message-Id: <1512534521-14760-1-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16039.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 4229243563bcb22990f71d50e25be9ea6d44f519)