Support for suspendable read in VFS and libC

The support has two parts. First, a VFS plugin now gets passed an
I/O-response handler callback on construction, which informs users of the
VFS that an I/O event occurred. This enables, for example, the libC to
check if blocking read can be completed. Further, the VFS file I/O
interface provides now functions for suspendable reads, i.e.,
queue_read() and complete_read().

repos/os/src/lib/vfs/null_file_system.h

@@ -24,7 +24,8 @@ struct Vfs::Null_file_system : Single_file_system
 {
 	Null_file_system(Genode::Env&,
 	                 Genode::Allocator&,
-	                 Genode::Xml_node config)
+	                 Genode::Xml_node config,
+	                 Io_response_handler &)
 	:
 		Single_file_system(NODE_TYPE_CHAR_DEVICE, name(), config)
 	{ }