Fix bug 824126: Conditionally redirect /ports/os2

To test that this works correctly, ensure that the following redirects work (using regex pseudo-code here).

1. `mozilla.org/qtmozilla(optional /)`→`https://wiki.mozilla.org/Qt`
2. `mozilla.org/ports/os2(optional /)`→`https://wiki.mozilla.org/Ports/os2`
3. `mozilla.org/ports/os2/gcc(anything-or-nothing-else)`→`404 (intentionally)`
4. `mozilla.org/ports/os2/selmail(anything-or-nothing-else)`→`404 (intentionally)`
5. `mozilla.org/ports(anything-or-nothing-else)`→`http://www-archive.mozilla.org/ports(value-of-"anything-or-nothing-else"-from-before)`