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/pipe.cpp

@@ -40,7 +40,7 @@ KResultOr<int> Process::sys$pipe(int pipefd[2], int flags)
         return EINVAL;
 
     u32 fd_flags = (flags & O_CLOEXEC) ? FD_CLOEXEC : 0;
-    auto fifo = FIFO::create(m_uid);
+    auto fifo = FIFO::create(uid());
 
     auto open_reader_result = fifo->open_direction(FIFO::Direction::Reader);
     if (open_reader_result.is_error())