Kernel: Consolidate timeout logic

Allow passing in an optional timeout to Thread::block and move
the timeout check out of Thread::Blocker. This way all Blockers
implicitly support timeouts and don't need to implement it
themselves. Do however allow them to override timeouts (e.g.
for sockets).

Kernel/Syscall.cpp

@@ -179,7 +179,7 @@ void syscall_handler(TrapFrame* trap)
     current_thread->die_if_needed();
 
     if (current_thread->has_unmasked_pending_signals())
-        (void)current_thread->block<Thread::SemiPermanentBlocker>(Thread::SemiPermanentBlocker::Reason::Signal);
+        (void)current_thread->block<Thread::SemiPermanentBlocker>(nullptr, Thread::SemiPermanentBlocker::Reason::Signal);
 }
 
 }