supervise-daemon: don't overwrite empty supervise_daemon_args

If supervise_daemon_args is not set *or empty*, it defaults to
`start_stop_daemon_args`. This is bad because supervise-daemon doesn't
accept the same options as `start-stop-daemon`. So if we set e.g.
`start_stop_daemon_args="--wait 50"`, but not `supervise_daemon_args`,
and the user adds `supervisor=supervise-daemon` to the corresponding
/etc/conf.d/<service> file, the service will fail to start due to
unrecognized option "wait".

It would be best to remove this fallback, but that might break some
existing scripts that depend on it. So this commit just changes it to
use `start_stop_daemon_args` as the default for `supervise_daemon_args`
only if `supervise_daemon_args` is not set at all, but not if it's
empty.

This at least simplifies workarounds; we can just add
`supervise_daemon_args="$supervise_daemon_args"` to init scripts.

This fixes #558.

sh/supervise-daemon.sh

@@ -41,7 +41,7 @@ supervise_start()
 		${no_new_privs:+--no_new_privs} \
 		${command_user+--user} $command_user \
 		${umask+--umask} $umask \
-		${supervise_daemon_args:-${start_stop_daemon_args}} \
+		${supervise_daemon_args-${start_stop_daemon_args}} \
 		$command \
 		-- $command_args $command_args_foreground
 	rc=$?