Fix mail subject switch (#1710)

Signed-off-by: Lantrix <lantrix@pobox.com>

lib.sh

@@ -1531,7 +1531,7 @@ send_mail() {
         if [ -n "$RECIPIENT" ]
         then
             print_text_in_color "$ICyan" "Sending '$1' to $RECIPIENT"
-            if echo -e "$2" | mail --subject "NcVM - $1" "$RECIPIENT"
+            if echo -e "$2" | mail -s "NcVM - $1" "$RECIPIENT"
             then
                 return 0
             fi