Kernel+LibPthread+LibC: Add a naive futex and use it for pthread_cond_t

This patch implements a simple version of the futex (fast userspace
mutex) API in the kernel and uses it to make the pthread_cond_t API's
block instead of busily sched_yield().

An arbitrary userspace address is passed to the kernel as a "token"
that identifies the futex and you can then FUTEX_WAIT and FUTEX_WAKE
that specific userspace address.

FUTEX_WAIT corresponds to pthread_cond_wait() and FUTEX_WAKE is used
for pthread_cond_signal() and pthread_cond_broadcast().

I'm pretty sure I'm missing something in this implementation, but it's
hopefully okay for a start. :^)

Kernel/UnixTypes.h

@@ -46,6 +46,9 @@
 
 #define FD_CLOEXEC 1
 
+#define FUTEX_WAIT 1
+#define FUTEX_WAKE 2
+
 /* c_cc characters */
 #define VINTR 0
 #define VQUIT 1