ref: 31984582c702a9febde8773e1e24f305c4eceae4
parent: 8d59ff3b29f759e550dc421b8cd14d96b83b5001
author: Sigrid Solveig Haflínudóttir <ftrvxmtrx@gmail.com>
date: Tue Mar 16 09:21:35 EDT 2021
move exit(3) to plan9.h, choose between threadexitsall and exits depending on __plan9_keep_main__