sl0ppty: let a child's exit status survive long enough to be read

The server set `signal(SIGCHLD, SIG_IGN)`, which is not the same as having
no handler. SIG_IGN asks the kernel to reap children itself and throw their
statuses away, so `waitpid` can never answer: every pane's program exited
with "no idea".

Nothing read a status yet, so nothing was visibly wrong -- and the headless
driver, which is what the whole test suite drives, never set it at all. A
suite that is green either way is a suite that would never have caught this.

SIG_DFL is what was actually wanted: SIGCHLD's default action is to be
discarded, so nothing is interrupted and no handler is needed, but the child
waits as a zombie until somebody asks how it went.

That makes statuses readable and zombies possible, so pty_close now collects
the child it just hung up rather than glancing once with WNOHANG -- bounded
to a few hundred microseconds, because a process that outlives a SIGHUP is a
larger problem than the process-table entry it leaves behind.

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