[Usability] Server side: auto wait the child and exit 30min after pty/slave closes and child exits

[clarkwang]

This would help avoid too many zombies running around.

New option:

• spawn -zombie-ttl N (N=-1 for infinity; Default: 10min? 30min?)
• get -zombie-ttl? Not very necessary. get -all should be enough.
• set -zombie-ttl? Don't bother.