Cleanup in SpecialPage.php:

Cleanup in SpecialPage.php:
* Enforce private access for member variables suggested since at least 1.4.  Didn't do $mName because grepping for "->mName" gave far too many results to check.
* Move the stuff related to redirects (getRedirect(), getRedirectQuery(), $mAllowedRedirectParams and $mAddedRedirectParams) to SpecialRedirectToSpecial and adjust callers
* Document stuff
* Mark getFile() as deprecated
* Group together getListed(), setListed() and listed() to draw attention to the fact that all three have been there since 1.6 and that we need to pick one and deprecate the other(s)
* add isIncludable() getter
* mark as deprecated and evil the mutators added in 1.6 for things which *really* shouldn't be mutating anywhere.  AFAICT they're not actually used many places.  Didn't deprecate including() as it's in wide use and it's legitimately set in SpecialPageFactory::executePath().