Fix bug 824126: Conditionally redirect /ports/os2.

To test that this works correctly, ensure that the following redirects work (using regex pseudo-code here).

* `mozilla.org/qtmozilla(optional /)`→`https://wiki.mozilla.org/Qt`
* `mozilla.org/ports/os2(optional /)`→`https://wiki.mozilla.org/Ports/os2`
* `mozilla.org/ports/os2/gcc(optional /)`→`404 (intentionally)`
* `mozilla.org/ports/os2/selmail(optional /)`→`404 (intentionally)`
* `mozilla.org/ports(something-else)`→`http://www-archive.mozilla.org/ports$something-else`