ptrace: Add PT_SETREGS

PT_SETTREGS sets the regsiters of the traced thread. It can only be
used when the tracee is stopped.

Also, refactor ptrace.
The implementation was getting long and cluttered the alraedy large
Process.cpp file.

This commit moves the bulk of the implementation to Kernel/Ptrace.cpp,
and factors out peek & poke to separate methods of the Process class.

Kernel/Thread.cpp

@@ -944,4 +944,10 @@ void Thread::tracer_trap(const RegisterState& regs)
     send_urgent_signal_to_self(SIGTRAP);
 }
 
+const Thread::Blocker& Thread::blocker() const
+{
+    ASSERT(m_blocker);
+    return *m_blocker;
+}
+
 }