Call _exit() instead of exit() if exec*() fails

exit() will also unwind the atexit() functions.  This is bad
because if exec*() fails the process is in an inconsistent state.
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