* Rewrote wfArrayMerge() in terms of array plus

* Rewrote wfArrayMerge() in terms of array plus
* Two callers of wfArrayMerge() were bugs, both assuming strange and complex behaviour in wfArrayMerge() which has never been present or documented.
* Introduced wfMergeErrorArrays() to remove duplicates from merged error arrays, e.g. from getUserPermissionsErrors().
* Rewrote the remaining callers of wfArrayMerge() to use array plus. It makes the code clearer, assuming the reader knows more about basic PHP operators than GlobalFunctions.php. Considering the two bugs discussed above, this seems like a fair assumption. If you don't know PHP, you shouldn't be writing MediaWiki code.