Change /etc/mtab to /proc/mounts to enable running on busybox system
Note: This patch was originally submitted by Patrik in November, 2008.
However, after some review comments were provided back to Patrik, there
was no response, so the patch was never committed. A new request for
this fix arrived on the list on May 21, 2013 from Mark Pearson, so
we revive this patch and commit it now.