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/FileSystem/FIFO.cpp

@@ -95,6 +95,11 @@ FIFO::FIFO(uid_t uid)
     LOCKER(all_fifos().lock());
     all_fifos().resource().set(this);
     m_fifo_id = ++s_next_fifo_id;
+
+    // Use the same block condition for read and write
+    m_buffer.set_unblock_callback([this]() {
+        evaluate_block_conditions();
+    });
 }
 
 FIFO::~FIFO()
@@ -116,6 +121,8 @@ void FIFO::attach(Direction direction)
         klog() << "open writer (" << m_writers << ")";
 #endif
     }
+
+    evaluate_block_conditions();
 }
 
 void FIFO::detach(Direction direction)
@@ -133,6 +140,8 @@ void FIFO::detach(Direction direction)
         ASSERT(m_writers);
         --m_writers;
     }
+
+    evaluate_block_conditions();
 }
 
 bool FIFO::can_read(const FileDescription&, size_t) const