[Documentatin][Enhancement] Add PAM return codes emitted by pam_u2f to documentation

[nagmat84]

Please add which returns codes pam_u2f can emit to the documentation of pam_u2f at https://developers.yubico.com/pam-u2f/Manuals/pam_u2f.8.html and to the man pages. This should also contain a brief description what return code pam_u2f used in which case.

From reading the main source file I was able to compile this preliminary list:

• PAM_IGNORE:
  • unable to allocate memory (side remark: shouldn't this be PAM_ABORT or PAM_BUF_ERROR?)
  • could not resolve authfile path
  • unable to switch user to uid
  • could not restore privilege
• PAM_CONV_ERR
  • unable to get user name from PAM
• PAM_USER_UNKNOWN
  • unable to retrieve credentials for user
• PAM_AUTHINFO_UNAVAIL`
  • failed to perform variable expansion
  • unable to get devices from authentication file
  • found no devices
• PAM_SUCCESS
  • found no devices but nouserok specified
• PAM_AUTH_ERR`

Note: This list incomplete, because I did not dive into the helper functions get_devices_from_authfile, do_authentication and do_manual_authentication. I compiled this list only from the main file pam-u2f.c.

[LDVG]

Thank you for your suggestion. I also believe we could do better in these regards. We'll look into it when we get the time, patches welcome.