Save advanced search namespace prefs on Special:Search itself

* Checkbox on own row below power search checkboxes per MatmaRex;
  avoiding a mw-search-ns* id leaves it untouched by All/None JS.
* The option searcheverything is removed: a "shortcut" which is no
  longer necessary now that options can be (un)selected at once
  with All/None buttons on search page itself.
* Require a token for saving: no accidental preferences changes.
* Keep the searchoptions/advancedsearchoptions prefs section in case
  something is using it (no known extension does though); options
  are converted to "api" type so it's empty and hidden by default.
* Add minimal documentation for saveSettings() and friends
  (@todo since 155ddf6de, 2009!).

Bug: 52817
Change-Id: I514cee835988600cc013658049e88a10b670e64a