listen-like-systemd -- Open a listening socket and pass it to subprocess

This program performs the parent side of systemd's socket activation dance. This is useful for e.g. development, where you want to run a program like it was socket activated, but not create a systemd service for it.

listen-like-systemd has pretty much been obsoleted by systemd's own systemd-socket-activate command, though I still prefer the calling convention used here.

Bugs

can overwrite actively used fds 3, 4, ...

• systemd design forces passed fds to be numbered starting from 3
• Go runtime on BSD derivatives uses an fd around 3-5 to hold open kqueue fd
• there's a race in where runtime could try to use the supposedly-kqueue fd after our dup2
• could push dup2 later, to minimize window; seems impossible to actually fix, with pure Go
• could fork and leave parent running, and avoid the issue by using https://pkg.go.dev/os#ProcAttr.Files to adjust the FD numbers, but leaving the parent around is ugly and would mess up LISTEN_PID