(internal_getgrsid): Only refresh if uninitialized.
(internal_getgrent): Ditto.
* passwd.cc (getpwent): Only refresh passwd entries when at beginning.
(pwdgrp::read_passwd): linebuf *cannot* be NO_COPY.
(internal_getpwsid): Only refresh if uninitialized.
(getpass): No need to refresh passwd data here.
* pwdgrp.h (refresh): Eliminate default.