Kernel+Userland: Remove the {get,set}_thread_name syscalls

These syscalls are not necessary on their own, and they give the false
impression that a caller could set or get the thread name of any process
in the system, which is not true.

Therefore, move the functionality of these syscalls to be options in the
prctl syscall, which makes it abundantly clear that these operations
could only occur from a running thread in a process that sees other
threads in that process only.

Kernel/Tasks/Process.cpp

@@ -639,6 +639,19 @@ size_t Process::OpenFileDescriptions::open_count() const
     return count;
 }
 
+ErrorOr<NonnullRefPtr<Thread>> Process::get_thread_from_thread_list(pid_t tid)
+{
+    if (tid < 0)
+        return ESRCH;
+    return m_thread_list.with([tid](auto& list) -> ErrorOr<NonnullRefPtr<Thread>> {
+        for (auto& thread : list) {
+            if (thread.tid() == tid)
+                return thread;
+        }
+        return ESRCH;
+    });
+}
+
 ErrorOr<Process::ScopedDescriptionAllocation> Process::OpenFileDescriptions::allocate(int first_candidate_fd)
 {
     for (size_t i = first_candidate_fd; i < max_open(); ++i) {