Kernel: Some futex improvements

This adds support for FUTEX_WAKE_OP, FUTEX_WAIT_BITSET, FUTEX_WAKE_BITSET,
FUTEX_REQUEUE, and FUTEX_CMP_REQUEUE, as well well as global and private
futex and absolute/relative timeouts against the appropriate clock. This
also changes the implementation so that kernel resources are only used when
a thread is blocked on a futex.

Global futexes are implemented as offsets in VMObjects, so that different
processes can share a futex against the same VMObject despite potentially
being mapped at different virtual addresses.

Kernel/Forward.h

@@ -37,6 +37,7 @@ class DiskCache;
 class DoubleBuffer;
 class File;
 class FileDescription;
+class FutexQueue;
 class IPv4Socket;
 class Inode;
 class InodeIdentifier;