Remove log when /var/cache/inadyn is missing and no $HOME

[martinetd]

If the service uses the default /var/cache/inadyn directory, then that
directory will have been created and there is no problem, but if using
an alternate directory with --cache-dir is used then it must also be
passed to any --check-config invocation as well lest this error message
is printed:

Cannot create fallback cache dir: No such file or directory

However, there already is another message that is only printed on
service start and not on check-conf:

inadyn[1902671]: No write permission to /var/cache/inadyn: Permission denied
inadyn[1902671]: Cannot guarantee DDNS server won't lock you out for excessive updates.

So just remove the 'Cannot create fallback' message.

---

This is a follow-up on #522 -- sorry for bugging you on a closed PR there!
You've suggested adding an extra fallback to /tmp, and I first did that, but when writing the commit message at this point I think we're better off without the extra fallback:

• I think there's already too many fallbacks; it's a matter of like but I prefer things to fail hard than to try too hard in non-optimal situation
• the cache dir is meant to avoid spamming upstream services on restart but /tmp will likely be gone after reboot, so it'd just give a false impression of working (although it'll still work for service restarts, so I guess it's better than nothing?)
• since the cache dir being missing isn't a fatal error, I think it's better to have inadyn print many warnings everytime the cache dir is used than fallback and mask the error behind a single warning.. Although at this point I guess it's up to the user to fix their setup anyway.

If you think a fallback is better I have a commit here[1] that just needs message fixing a bit, so just tell me and I'll swap it in this PR.
[1] martinetd@06d61f1

Thanks,

[troglobit]

You make a good point, and I agree, there are too many fallbacks. So this approach is sane, but I'd like one-up you and go even further. How about this?

    home = getenv("HOME");
    if (!home) {
        logit(LOG_ERR, "%s: not writable, $HOME missing, please use --cache-dir=PATH", cache_dir);
        free(cache_dir);
        return 1;        <-- Fail in DO() so we force exit.
    }

[martinetd]

I can only approve of this :)

This still has to be conditional on dryrun so --check-config doesn't fail (unless you think --check-config needs to pass --cache-dir as well), I've updated accordingly with a comment saying why we skip this for check-config.

Tested with env -i inadyn -n (or without the -n, this fails before forking)

[troglobit]

lgtm, nice work on this, and thank you for being patient! 😎