Report a pty hangup on every path that can observe one - #278

Report a pty hangup on every path that can observe one

epoll_wait never reported EPOLLHUP for a pty master whose slaves had all
closed, and slaves obtained through TIOCGPTPEER were never registered, so
the master's hangup accounting missed them. Wire both, and re-verify the
fd generation a registration pinned at ADD/MOD so a close-and-reuse in the
lookup window cannot charge the hangup to an unrelated file.

Drop proc_pty_any_master_hung_up(). It was meant as a cheap gate that let
epoll skip the registration sweep, but it latches: a master that hangs up
while the guest still holds its fd keeps the gate true for the process's
lifetime, and every epoll_pwait then paid for a full FD_TABLE_SIZE sweep.
Track per-instance active and pty-master counts instead, so the scan is
bounded by what the instance actually registered and skipped entirely when
it holds no pty master.

Delete kqueue events whose guest fd is no longer registered. A
level-triggered event for a deregistered fd is re-reported on every call
and translates to zero epoll events, which spins the caller.

Keep an epoll registration alive while another fd still refers to the same
open file description, matching Linux, and identify that description by
ofd_id at the close chokepoint.

Return ready events in preference to EINTR. Linux ep_poll() checks
ep_events_available() and jumps to send_events before it consults
signal_pending, so EINTR is only the answer for a wait that produced
nothing. Discarding a ready fd to report EINTR loses it in practice: kqueue
re-reports it, but the same pending signal is still there, so the caller is
handed EINTR forever and never drains the fd. A terminal with an unhandled
SIGCHLD spun at 100% CPU this way without ever drawing.

Fix #274

Summary by cubic

Make pty hangups visible on every path and across processes. poll, epoll, read, and readv now report hangups reliably; epoll stamps EPOLLHUP immediately and reads return EIO after draining pending data.

• Bug Fixes
  • Per-pty shared counters via named shm (fresh on new host pty, join on dup/SCM_RIGHTS/fork-restore; fallback to per-process). Release this process’s remaining slaves at teardown; parent credits inherited slaves before siblings run; child adopts inherited slaves on init.
  • Track slaves opened via TIOCGPTPEER (only if the master generation matches) and mirror dup/dup2 of pty slaves so aliases are counted.
  • Pin fd generations across read/poll/epoll/ioctl to avoid close+reuse races; expose fd_current_generation; resolve host fd and generation atomically.
  • Epoll: stamp EPOLLHUP immediately (no waiting), merge with EPOLLIN when data is pending, bound scan using per-instance active and pty-master counts, honor EPOLLONESHOT, verify registration generation to avoid DEL/ADD races, keep registrations alive while another fd refers to the same open file description (track ofd_id), drop kqueue events for deregistered fds, and return ready events in preference to EINTR.
  • read and readv on hung-up masters return EIO after draining queued data.

^{Written for commit 26de80c. Summary will update on new commits.}