Allow command specific options in sites/default/drushrc.php to work consistently with rsync

If you have the following definition in sites/default/drushrc.php:

$options['command-specific']['rsync'] = array(
  'delete' => true,
$options['target-command-specific']['rsync'] = array(
  'delete' => true,
);

Then --delete will be added to your rsync options if your site is bootstrapped (e.g., if running drush rsync from your sites directory, but will not be added if the site is merely referenced in the rsync command (e.g. drush rsync path @site).

The patch allows the command-specific options to be applied uniformly in both scenarios. (See below for patch.)

Update: 'command-specific' is used when the site is bootstrapped, 'source-command-specific' is used when the site is referenced in the source parameter of rsync, and 'target-command-specific' is used when the site is referenced in the target parameter of rsync. This is consistent with what is done today for command-specific options in an alias record.