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 "csx < csmax" failed: file "../../../src/oldclasslib/include/charsink.h", line 162, function "virtual character_sink& string_accumulator::operator<<(int)"
[1]   Abort trap              ../../examples/m...
Exit 134