Thread: Set m_blocker to null in Thread::unblock()

Before this commit, m_blocker was only set to null in Thread::block,
after the thread has been unblocked.

Starting with this commit, m_blocker is also set to null in
Thread::unblock.

This change will allow us to implement a missing feature of the PT_TRACE
command of the ptrace syscall - stopping the traced thread when it
exits the execve syscall.

That feature will be implemented by sending a blocking SIGSTOP to the
traced thread after it has executed the execve logic and before it
starts executing the new program in userspace.

However, since Process::exec arranges the tss to return to userspace
(the so-called "yield-teleport"), the code in Thread::block that should
be run after the thread unblocks, and sets m_blocker to null, never
actually runs.

Setting m_blocker to null in Thread::unblock allows us to avoid an
incorrect state where the thread is in a Running state but conatins a
pointer to a Blocker.

Kernel/Thread.cpp

@@ -163,6 +163,7 @@ Thread::~Thread()
 
 void Thread::unblock()
 {
+    m_blocker = nullptr;
     if (current == this) {
         if (m_should_die)
             set_state(Thread::Dying);