LibC: Deprecate strcpy(), strncpy(), strcat() and strncat() :^)

And also mark strlcpy() and strlcat() with __attribute__((warn_unused_result)).

Since our code is warning-free, this ensures we never misuse those functions.
(Or are very sure about doing it when turning off the warning for a particular
piece of code.)

Userland/Tests/Kernel/elf-symbolication-kernel-read-exploit.cpp

@@ -37,6 +37,8 @@ asm("haxcode:\n"
 extern "C" void haxcode();
 extern "C" void haxcode_end();
 
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+
 int main()
 {
     char buffer[16384];