ahci/libdriver: multithreading support

This patch adds support for executing multiple concurrent requests on
different devices on the same AHCI controller. The libdriver library
has been extended to include a generic multithreading interface, and
the AHCI driver has been extended to make use of this interface.

The original version of this code has been written by Arne Welzel.

drivers/hello/hello.c

@@ -38,7 +38,8 @@ PRIVATE struct driver hello_tab =
     nop_cancel,
     nop_select,
     nop_ioctl,
-    do_nop,
+    NULL,
+    NULL
 };
 
 /** Represents the /dev/hello device. */