No AIX/IBM i PASE support (forkpty/openpty missing, but Unix98 primitives work) #963

forkpty(3)/openpty(3) (BSD convenience wrappers) don't exist on AIX/PASE — no header declares them. However, the underlying POSIX Unix98 primitives (posix_openpt, grantpt, unlockpt, ptsname) are present and fully functional (confirmed with a standalone test: opening a pty, forking, setsid() + open() of the slave without O_NOCTTY is enough to acquire it as the controlling terminal — no TIOCSCTTY needed, and it isn't even defined on this platform).

Proposed fix: an #elif defined(_AIX) branch providing pase_forkpty()/pase_openpty() built on those primitives, wired into the two call sites in PtyFork/PtyOpen, plus removing -lutil from the link step for OS=="os400" in binding.gyp (that library doesn't exist on PASE and isn't needed once forkpty/openpty aren't used directly).

Validated end-to-end on IBM i 7.5 PASE (gcc-12, Node 22): compiled cleanly, spawned a real bash shell through node-pty's JS API, confirmed interactive echo (readline), resize(), and the exit-code callback all work correctly. Also confirmed live through the actual RemoteForI Server (a fork of openvscode-server) in a real browser.

Happy to open a PR with the full patch if useful.