telnet: don't ignore HAVE_OPENPTY on linux

`openpty()` is not available on all Linux distributions. Trust autoconf's determination for `HAVE_OPENPTY` instead of unconditionally using `openpty()` on all Linux.

This is similar to the change to roken in 9ae257d7d444667c9cb368bb95f744ae00240b59, but this is for telnet.

This is a 1.6-only change, since telnet has been removed from master in e55b0d0ca5038a8101276a593ffbb6be4c27c8d0.