Conform sentence spacing in code comments.

The attached patch conforms *all* the sentence spacing in code comments to a single space following the period. Note that without this patch, core is very inconsistent; it mostly conforms to this standard but the size of this patch no doubt indicates how often we deviate from that standard. If this is committed, I'll edit the coding standards page and add an entry for one-space sentence spacing.

Please, please, someone commit this path before it no longer applies. If anything is changed in here other than sentence spacing, it was a mistake.