replace exit(3) by _exit(2) in signal handler sigchld()

exit(3) is not async-signal-safe but, _exit(2) is.
This change prevents st to crash and dump core.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea and freedom
500 Internal Server Error

Gitea Version: 1.28.0+dev-505-g31b4d79a84