Bug 761917 (GLUSTER-185)

Since it looks like the GlusterFS client supports supports adding backup --volfile-server options please support this in mount.glusterfs if not already done so.  The result will remove this as a point of failure in high availability setups when multiple servers can be used to hand out the identical volfile. 

Diff below is against GlusterFS 2.0.6rc2.
106a107,110
>     if [ -n "$backupvolfile_server" ]; then
>         cmd_line=$(echo "$cmd_line --volfile-server=$backupvolfile_server");
>     fi
>     
134a139,140
>     backupvolfile_server=$(echo "$options" | sed -n 's/.*backupvolfile-server=\([^,]*\).*/\1/p');
> 
162a169
>         -e 's/[,]*backupvolfile-server=[^,]*//' \