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.cpp

@@ -1,9 +1,17 @@
 #include <AK/Assertions.h>
+#include <AK/String.h>
 #include <AK/Types.h>
 #include <Kernel/Arch/i386/CPU.h>
 #include <Kernel/Heap/kmalloc.h>
 #include <Kernel/StdLib.h>
 
+String copy_string_from_user(const char* user_str, size_t user_str_size)
+{
+    SmapDisabler disabler;
+    size_t length = strnlen(user_str, user_str_size);
+    return String(user_str, length);
+}
+
 extern "C" {
 
 void* copy_to_user(void* dest_ptr, const void* src_ptr, size_t n)