srogue: add and use md_fdopen().

The implementation was copied from rogue4.  Using fdopen() is necessary
because the scorefile needs both encread() and encwrite().  For some
reason I have failed to discover, one of them uses FILE *'s and the
other uses file descriptors.

srogue/mdport.c

@@ -358,6 +358,16 @@ md_unlink(char *file)
 #endif
 }
 
+FILE *
+md_fdopen(int fd, char *mode)
+{
+#ifdef _WIN32
+    return( _fdopen(fd, mode) );
+#else
+    return( fdopen(fd, mode) );
+#endif
+}
+
 int
 md_chmod(const char *filename, int mode)
 {