1. finished porting wxDir/FileDialog to MS-DOS

2. added code to dynamically enable/disable controls in wxFileDialog depending on which
   actions are permitted at the moment (e.g. you can't click the 'Up' button if
   you're at / directory


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@13494 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/generic/dirctrlg.cpp

@@ -317,7 +317,7 @@ static const char * icon8_xpm[] = {
 
 #if defined(__DOS__)
 
-static bool wxIsDriveAvailable(const wxString dirName)
+bool wxIsDriveAvailable(const wxString& dirName)
 {
     if ( dirName.Len() == 3 && dirName[1u] == wxT(':') )
     {
@@ -363,7 +363,7 @@ int setdrive(int drive)
 #endif // !GNUWIN32
 }
 
-static bool wxIsDriveAvailable(const wxString dirName)
+bool wxIsDriveAvailable(const wxString& dirName)
 {
 #ifdef __WIN32__
     UINT errorMode = SetErrorMode(SEM_FAILCRITICALERRORS | SEM_NOOPENFILEERRORBOX);