Kernel: Move select Process members into protected memory

Process member variable like m_euid are very valuable targets for
kernel exploits and until now they have been writable at all times.

This patch moves m_euid along with a whole bunch of other members
into a new Process::ProtectedData struct. This struct is remapped
as read-only memory whenever we don't need to write to it.

This means that a kernel write primitive is no longer enough to
overwrite a process's effective UID, you must first unprotect the
protected data where the UID is stored. :^)

Kernel/Syscalls/process.cpp

@@ -32,7 +32,7 @@ namespace Kernel {
 KResultOr<pid_t> Process::sys$getpid()
 {
     REQUIRE_PROMISE(stdio);
-    return m_pid.value();
+    return pid().value();
 }
 
 KResultOr<pid_t> Process::sys$getppid()