Kernel: PID/TID typing

This compiles, and contains exactly the same bugs as before.
The regex 'FIXME: PID/' should reveal all markers that I left behind, including:
- Incomplete conversion
- Issues or things that look fishy
- Actual bugs that will go wrong during runtime

Kernel/ThreadTracer.cpp

@@ -32,7 +32,7 @@
 
 namespace Kernel {
 
-ThreadTracer::ThreadTracer(pid_t tracer_pid)
+ThreadTracer::ThreadTracer(ProcessID tracer_pid)
     : m_tracer_pid(tracer_pid)
 {
 }