Fixes for r28797.

Fixes for r28797.
* Mark private methods private using a keyword.
* Reject arrays with count == 2: these will fail when you do array_slice( ... , 1 ).
* Treat xor consistent with the other operations: if there's only one parameter the result should just evaluate that, not always return false; and any number of parameters should be allowed.
* Fail fast on bad input: throw an exception if Autopromote encounters a condition it can't understand (after asking extensions).
* Code documentation!  There were five lines of comments in the original commit.
* APCONDS_INGROUPS is not used, or for that matter defined.
* Editcount should use >=, not >, for consistency with past behavior and intuitiveness.
* "autopromoteUser" sounds like it's actually promoting the user somehow.  Renamed the function to getAutopromoteGroups.
* Make sure we don't return the same group more than once, when we're returning a group.  Probably not going to hurt, but may as well be clean.