Site aliases should load the drushrc.php config file for the site selected

This patch loads the drushrc.php config file found in the 'site root' folder for any local (non-remote) site that is specified. If, for example, you have an alias called 'mysite1' and another called 'mysite2', and each has a drushrc.php file that defines (different) site aliases named 'stage', then you could use drush sql-sync mysite1 stage or drush sql-sync mysite2 stage, and have the correct stage site be selected for the particular dev site being used.

Note that the reverse does not work; drush sql-sync stage mysite1 cannot be used, because sql-sync looks for the 'stage' alias before it looks for the 'mysite1' alias, so 'stage' is not defined soon enough to be used. A lot of people recommend 'one-way sync-ing' of Drupal sites, though, so this is not a terrible limitation.