Optimized include/*.h: (recursively) include all headers needed by
this .h file, but only those. Necessary fixes to a lot of .c files,
started optimizing "windows.h" away from some of them. Moved
GetCurrentTask prototype to wine/winbase16.h.

diff --git a/files/file.c b/files/file.c
index 9768cac..c596564 100644
--- a/files/file.c
+++ b/files/file.c
@@ -24,8 +24,11 @@
 #include <unistd.h>
 #include <utime.h>
 
-#include "windows.h"
 #include "winerror.h"
+#include "windef.h"
+#include "winbase.h"
+#include "wine/winbase16.h"
+#include "wine/winestring.h"
 #include "drive.h"
 #include "device.h"
 #include "file.h"