[RFE] OpenBSD authentication support

Could we see OpenBSD authentication support as `login_vault` ?
See http://man.openbsd.org/login.conf#AUTHENTICATION and probably http://man.openbsd.org/auth_subr.3.

Source to various based OpenBSD `login_*` authentication "modules" (in C) could be found here: https://github.com/openbsd/src/tree/master/libexec.

Although OpenBSD does not support PAM and `sshd` does 'password' authentication itself, it seems that `sshd` could talk to *bsdauth* via setting `AuthenticationMethods keyboard-interactive:bsdauth` options, https://man.openbsd.org/sshd_config#AuthenticationMethods. Not tested by me.

Start with the OpenBSD login.conf and auth_subr documentation, then inspect the OpenBSD libexec login_* sources and the login_vault and sshd AuthenticationMethods references. Determine the integration needed for bsdauth; done means OpenBSD authentication support is implemented and works through the described sshd configuration.

Written by the indexing model from the issue text.