IsFunc() - check if function exist by string

[mLipok]

IsFunc() should have feautre to check if string is reperesenting FunctionName in function list.

This feature is very much needed to validate user function parameters.

In this following example only third test should fail but second should display message.

_Test(_msg, 1)
_Test("_msg", 2)
_Test("Fake_msg", 3)

Func _Test($sFunction, $iTest)
	If IsFunc($sFunction) Then $sFunction($iTest)
EndFunc   ;==>_Test1

Func _Msg($iTest)
	MsgBox(0, "Test", $iTest)
EndFunc   ;==>_Msg

Remarks:
I think that, "Return values" should not need to be changed.

This is even more necessary if user want to use external tools to obfuscate code like Au3Stripper.