Change the display order of PAM messages

[Gliese852]

This is some suggestion to improve (in my opinion) the interaction between the user and the PAM module.

There are 3 test users on my computer, one just logs in with a password, the second one with the fingerprint scanner (pam_fprintd.so) and the third one with the help of howdy.

Howdy works great, but I noticed a somewhat awkward behavior:

• The first two users who are not using howdy see the message "There is no face model known", although they did not intend to use this method.
• Each user sees the message "Attempting facial authentication" if the corresponding option is active, including those, who don't need it.
• in case of unsuccessful authentication (by timeout), the user does not see any message.

This request represents my attempt to improve it.

Since the comparison takes place in a separate script, I decided to connect to its stdout and, upon receiving the HAS_MODEL string, prompt the user. Not sure if this is the best way, but it seems simple and painless enough.

P.S. I just brought the behavior closer to that of the pam_fprintd.so module, and it seems to me quite convenient - if the user has not configured a fingerprint, we silently skip it, if we have configured it, we write an invitation, if authentication is unsuccessful, we write an error message and exit.

[boltgolt]

Sorry for taking this long to take a look at this properly! I think we need to leave the config value to false for now, per my comment. There's also a linter warning in the file from the github action, i think that is a really simple change. Thank you for contributing code!

howdy/src/pam/main.cc:259:5: error: do not use 'else' after 'return' [readability-else-after-return,-warnings-as-errors]
  } else if (config.GetBoolean("core", "detection_notice", true)) {

[boltgolt]

Thank you!