Translation of long month names not as in core

As of Drupal 6, format_date uses the prefix !long-month-name when translating month names (more info). The Date module's date_t almost supports the distinction between long and short names, but does not use the !long-month-name prefix.
This has two unfortunate consequences:

1. In English "May" is both the long and short name for the fifth month. In other languages the name may be longer, and the translator must thus know from the context if the short or long local version should be used. Without the prefix, correct translations cannot always be made.
2. Because Drupal 6 core uses translation entries like "!long-month-name June" (long) and "Jun" (short), all translation files ("language packs") do so too. Language packs thus have no translations for long month names without prefix (e.g. "June"). It means dates printed by the Date module are not translated correctly (unless someone has made a translation for "June" etc. too).

The attached patch worked for me, and I expect it to work for others too.