Kernel: Move block condition evaluation out of the Scheduler

This makes the Scheduler a lot leaner by not having to evaluate
block conditions every time it is invoked. Instead evaluate them as
the states change, and unblock threads at that point.

This also implements some more waitid/waitpid/wait features and
behavior. For example, WUNTRACED and WNOWAIT are now supported. And
wait will now not return EINTR when SIGCHLD is delivered at the
same time.

Kernel/UnixTypes.h

@@ -71,6 +71,7 @@ enum {
 #define WSTOPPED WUNTRACED
 #define WEXITED 4
 #define WCONTINUED 8
+#define WNOWAIT 0x1000000
 
 #define R_OK 4
 #define W_OK 2
@@ -427,11 +428,12 @@ struct stat {
 };
 
 #define POLLIN (1u << 0)
-#define POLLPRI (1u << 2)
-#define POLLOUT (1u << 3)
-#define POLLERR (1u << 4)
-#define POLLHUP (1u << 5)
-#define POLLNVAL (1u << 6)
+#define POLLPRI (1u << 1)
+#define POLLOUT (1u << 2)
+#define POLLERR (1u << 3)
+#define POLLHUP (1u << 4)
+#define POLLNVAL (1u << 5)
+#define POLLRDHUP (1u << 13)
 
 struct pollfd {
     int fd;