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.
Merge pull request #66 from ktdreyer/openpty-1-6

telnet: don't ignore HAVE_OPENPTY on linux