PHP >= 5.4.0 started taking advantage of $limit optimization of wfDebugBacktrace(),

PHP >= 5.4.0 started taking advantage of $limit optimization of wfDebugBacktrace(),
which we weren't treating right.

* $limit in wfDebugBacktrace() is the number of returned frames,
we thus need to take into account the wfDebugBacktrace() frame, which
is sliced from debug_backtrace().

* wfGetCaller() needs to add a level for itself.

* MWDebug::warning() was logging itself as the warning issuer,
which is useless (the call a few lines before was right, though)
MWDebugTest.php changed accordingly.

* Removed double call to wfGetCaller( $callerOffset + 1 )

* Documented the meaning of wfGetCaller() parameter

* Added unit test

Change-Id: Ief50f4c810bad8b03bb2bf9dc6d945d9acb29851