[WFLY-2261] Fix standalone.sh --debug [<port>]

Fix standalone.sh script when the debug port is specified on the command
line.

WildFly's server Main class expects to get --debug from its args before
the debug port. Add it to the script's $SERVER_OPTS if debug mode is
set.

```
./standalone.sh 
  => no debug
./standalone.sh --debug
  => debug on default port 8787
./standalone.sh --debug 8888
  => debug on specified port 8888
```

JIRA: https://issues.jboss.org/browse/WFLY-2261