+terminating safety C<NUL> byte is appended (not included in the returned
+length).
+
+The error case is if the destination buffer is not large enough to accommodate
+everything that should be copied. In this situation, a value larger than
+S<C<to_end> - C<to>> is written to C<*retlen>, and as much of the source as
+fits will be written to the destination. Not having room for the safety C<NUL>
+is not considered an error.