pty.c: Fix errant use of fcntl F_SETFD (#951)

pty.c: Fix errant use of fcntl F_SETFD (#951)

* pty.c: Fix errant use of fcntl F_SETFL

When this was added in https://github.com/tsl0922/ttyd/commit/cfd338ea5e1a3c3023acade45980b3024c41e507

We before this time the *File descriptor flags* (`F_GETFD`/`F_SETFD`) were augmented to include `FD_CLOEXEC`
Then at this time the additional code added the *File status flags* (`F_GETFL`/`F_SETFL`) to include `O_NONBLOCK`, but this was weaved through code working with `F_SETFD` instead of `F_SETFL`. Some systems may use one big status word for all of them. Mine certainly dont.

FIXES #733