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/tar_file_system.h

@@ -344,7 +344,8 @@ class Vfs::Tar_file_system : public File_system
 
 		Tar_file_system(Genode::Env       &env,
 		                Genode::Allocator &alloc,
-		                Genode::Xml_node   config)
+		                Genode::Xml_node   config,
+		                Io_response_handler &)
 		:
 			_env(env), _alloc(alloc),
 			_rom_name(config.attribute_value("name", Rom_name())),