Kernel: Restore the previous thread state on SIGCONT after SIGSTOP

When stopping a thread with the SIGSTOP signal, we now store the thread
state in Thread::m_stop_state. That state is then restored on SIGCONT.
This fixes an issue where previously-blocked threads would unblock
upon resume. Now they simply resume in the Blocked state, and it's up
to the regular unblocking mechanism to unblock them.

Fixes #1326.

Kernel/Thread.h

@@ -486,6 +486,7 @@ private:
     u32 m_priority_boost { 0 };
 
     u8 m_stop_signal { 0 };
+    State m_stop_state { Invalid };
 
     bool m_dump_backtrace_on_finalization { false };
     bool m_should_die { false };