Update string concatenation to match Drupal 7 coding standards

In Drupal 7 our standards for string concatenation changed, from having a space on one side to having a space on both sides of periods when used for concatenation. We were already using the new standard for all new code in the 3.x branch in Drupal 6, but this patch completes all of the remain areas to fully comply with the code standards.

I've already committed this patch.