cli/exit: new package to replace direct uses of os.Exit

[jseldess]

Jesse Seldess commented:

Backported from 21.1 to 20.2.

From release notes:

- Some specific situations now have dedicated exit status codes. The following codes are defined:

    Code | Description
    -----|------------
    0 | Process terminated without error.
    1 | An unspecified error was encountered. Explanation should be present in the stderr or logging output.
    2 | Go runtime error, or uncaught panic. Likely a bug in CockroachDB. Explanation may be present in logging output.
    3 | Server process interrupted gracefully with Ctrl+C / SIGINT.
    4 | Command-line flag error.
    5 | A logging operation to the process' stderr stream failed (e.g. stderr has been closed). Some details may be present in the file output, if enabled.
    6 | A logging operation to file has failed (e.g. log disk full, no inodes, permission issue, etc). Some details may be present in the stderr stream.
    7 | Server detected an internal error and triggered an emergency shutdown.
    8 | Logging failed while processing an emergency shutdown.
    [#56574][#56574]

Jira Issue: DOC-958

[rmloveland]

We also have 10 - Disk Full via cockroachdb/cockroach#66893 which I am mentioning in an upcoming PR to fix #11156

This implies the existence of a 9 - Foo but IDK what it is