pty: Simplify ptsname call
pty: Simplify ptsname call

There's no need to use ptsname_r here or offer a fallback to
ioctl(TIOCGPTN), since all supported OSes (linux, BSDs, illumos)
have ptsname(3).

https://bugzilla.gnome.org/show_bug.cgi?id=747046

saitoha/vte-sixel,thestinger/vte-ng,thestinger/vte-ng,saitoha/vte-sixel,thestinger/vte-ng,saitoha/vte-sixel