get_new_macaddr: add 1 to the mac address instead of starting from 0x14

If someone have 2 adapters with the same hardware, there is a possibility
the first 5 bytes of their mac address to be the same.
I this case we will have mac address collision if we change both of them to
XX:XX:XX:XX:XX:14, one way to avoid this is to add 1 to their last byte.

Fix #47

create_ap

@@ -209,8 +209,9 @@ get_virt_iface_name() {
 
 get_new_macaddr() {
     OLDMAC=$(get_macaddr "$1")
-    for i in {20..255}; do
-        NEWMAC="${OLDMAC%:*}:$(printf %02x $i)"
+    LAST_BYTE=$(printf %d 0x${OLDMAC##*:})
+    for i in {1..255}; do
+        NEWMAC="${OLDMAC%:*}:$(printf %02x $(( ($LAST_BYTE + $i) % 256 )))"
         (ip link | grep "ether ${NEWMAC}" > /dev/null 2>&1) || break
     done
     echo $NEWMAC