readdir_r(3) was never necessary and has been deprecated by POSIX.
Document that in the manpage and stop using it internally.

ok deraadt@ millert@ jmc@

.Li -> .Vt where appropriate;
from josiah frentsos, tweaked by schwarze

ok schwarze

More correction of section 3 layout. directory->opendir, fts->fts_open,
getcap->cgetent. pwcache->user_from_uid. And then repair references.
ok jmc