Use memcpy instead of strncpy when we know the length anyway. Modern gcc seems to think it knows how to detect misuse of strncpy, but it's wrong (in fact: very, very wrong) and the path of least resistance is to not try to fight with it.

assertion "sx && lx < lxmax" failed: file "../../../src/oldclasslib/include/stack.h", line 121, function "stack<T>& stack<T>::operator<<(T) [with T = file_descriptor]"
[1]   Abort trap              ../../examples/m...
Exit 134