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/Syscalls/process.cpp

@@ -32,13 +32,13 @@ namespace Kernel {
 pid_t Process::sys$getpid()
 {
     REQUIRE_PROMISE(stdio);
-    return m_pid;
+    return m_pid.value();
 }
 
 pid_t Process::sys$getppid()
 {
     REQUIRE_PROMISE(stdio);
-    return m_ppid;
+    return m_ppid.value();
 }
 
 int Process::sys$set_process_icon(int icon_id)