[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.

JIRA: https://issues.jboss.org/browse/WFLY-2261