Added admin fields don't save when added to the permalinks page

[arickmann]

If you add a field to the permalinks page using add_settings_field, and register it, it doesn't save the value.

The same code will save the value perfectly well on the write page.

[ryan]

Unlike other settings pages, permalinks doesn't use the options.php POST handler. options-permalink.php would need to be extended to handle arbitrary options to fix this.

[ryan]

Postponing to 2.8.

[janeforshort]

Punting due to feature freeze. Reconsider with next release.

[ryan]

See #9296