CP_ACP (defaultsystemcodepage), because if all input strings have the
    same code page then the result should also have that code page if it's
    assigned to a rawbytestring rather than getting defaultsystemcodepage
  * do not consider empty strings to determine the code page of the result
    in fpc_AnsiStr_Concat_multi(), because that will cause a different
    result than when using a sequence of fpc_AnsiStr_Concat() calls (it
    ignores empty strings to determine the result code page) and it's also
    slower
  * do not consider the run time code page of the destination string in
    fpc_AnsiStr_Concat(_multi)() because Delphi does not do so either. This
    was introduced in r19118, probably to hide another bug
  + test

git-svn-id: branches/cpstrrtl@25143 -