modbus-rtu: don't use O_EXCL when opening the device

Quote from 'man 2 open':
-snip-
...
In general, the behavior of O_EXCL is undefined if it is used without O_CREAT.
...
-snap-

Since we don't create the device file here - we just want to open
(hopefully) existing ones - let's simply drop this flag to be on
the safe side.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

src/modbus-rtu.c

@@ -641,7 +641,7 @@ static int _modbus_rtu_connect(modbus_t *ctx)
 
        Timeouts are ignored in canonical input mode or when the
        NDELAY option is set on the file via open or fcntl */
-    flags = O_RDWR | O_NOCTTY | O_NDELAY | O_EXCL;
+    flags = O_RDWR | O_NOCTTY | O_NDELAY;
 #ifdef O_CLOEXEC
     flags |= O_CLOEXEC;
 #endif