Change WindowsError to carry the Win32 error code in winerror,

and the DOS error code in errno. Revert changes where
WindowsError catch blocks unnecessarily special-case OSError.

PC/generrmap.c

@@ -0,0 +1,20 @@
+#include <stdio.h>
+#include <errno.h>
+
+/* Extract the mapping of Win32 error codes to errno */
+
+int main()
+{
+	int i;
+	printf("/* Generated file. Do not edit. */\n");
+	printf("int winerror_to_errno(int winerror)\n");
+	printf("{\n\tswitch(winerror) {\n");
+	for(i=1; i < 65000; i++) {
+		_dosmaperr(i);
+		if (errno == EINVAL)
+			continue;
+		printf("\t\tcase %d: return %d;\n", i, errno);
+	}
+	printf("\t\tdefault: return EINVAL;\n");
+	printf("\t}\n}\n");
+}