nss_tacplus: Provide dummy setpwent(), getpwent_r(), endpwent().

These aren't really needed, since TACACS+ does not support enumeration, but providing placeholders keeps nsdispatch() from complaining that they're missing.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D41658

(cherry picked from commit 56b74a2d856c4d65a4b5c72d1352067b6b469d3b)