Kernel: Remove SmapDisablers in open(), openat() and set_thread_name()

This patch introduces a helpful copy_string_from_user() function
that takes a bounded null-terminated string from userspace memory
and copies it into a String object.

Kernel/StdLib.h

@@ -2,6 +2,12 @@
 
 #include <AK/Types.h>
 
+namespace AK {
+class String;
+}
+
+AK::String copy_string_from_user(const char*, size_t);
+
 extern "C" {
 
 static_assert(sizeof(size_t) == 4);