Adjust Linux+FreeBSD DCO device name handling to 'non DCO linux style'

On Linux, tun devices are created according to the following algorithm
  --dev tun    -> try tun0, tun1, ... tun255, use first free
  --dev anything -> create a TUN device named "anything"
(as long as "anything" is not "null" or "tap[N]")

DCO was following the "other platform convention", where everything
not having a digit was iterated ("--dev tun-home" -> "tun-home0") -
which does not work for classic tun/tap devices on the BSDs anyway,
so is not the best model.

Adjust open_tun_dco_generic() to document expected behaviour and
do the thing.

Signed-off-by: Gert Doering <[email protected]>
Acked-by: Kristof Provost <[email protected]>
Acked-by: Antonio Quartulli <[email protected]>
Message-Id: <[email protected]>
URL: https://www.mail-archive.com/[email protected]/msg25134.html
Signed-off-by: Gert Doering <[email protected]>

src/openvpn/tun.c

@@ -1943,12 +1943,14 @@ open_tun_dco_generic(const char *dev, const char *dev_type,
     }
 
     /*
-     * dynamic open is indicated by --dev specified without
-     * explicit unit number.  Try opening DCO device named "[dev]n"
-     * where n = [0, 255].
+     * unlike "open_tun_generic()", DCO on Linux and FreeBSD follows
+     * the device naming model of "non-DCO linux", that is:
+     *   --dev tun         -> try tun0, tun1, ... tun255, use first free
+     *   --dev <anything>  -> (try to) create a tun device named "anything"
+     * ("--dev tap" and "--dev null" are caught earlier and not handled here)
      */
 
-    if (!tun_name_is_fixed(dev))
+    if (strcmp(dev, "tun") == 0)
     {
         for (int i = 0; i < 256; ++i)
         {