nss_tacplus: Support setting the login class.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: thj
Differential Revision: https://reviews.freebsd.org/D40376

Add nss_tacplus, a TACACS+ NSS module.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D40133