SpecialPrefixindex formatting methods are now protected

namespacePrefixForm() and showPrefixChunk() accepted additional
arguments to slightly alter the form such as hiding redirects.  The
argument has been removed in favor of class property just like
stripprefix introduced earlier and the method have been made protected
since there is no point in calling them out of the special page scope.

Change-Id: I55728fd2634f8a935a033052dcce3c7247cb1aa3
Merge "SpecialPrefixindex formatting methods are now protected"