Child PID from `--info-fd` and `--json-status-fd` is not concurency safe

The bwrap main process will write the the JSON in to those options as soon as it forks:

Line 2922 in 8e51677

/* Parent, outside sandbox, privileged (initially) */

However, the child process still has a lot of initialization steps left.

This can result in a difficult to diagnose concurrency bugs. I experienced this then trying to write to the /proc/sys/user/max_* files. When you acquire a user namespace file descriptor from a child PID it can either be the direct child user namespace or a child user namespace with a intermediate parent depending on how far in to initialization the child is. (when using --dev option)

At least this concurrency should be documented in man page.