Prevent flex(1) from generating calls to isatty(3) - and, in turn,
TIOCGETA ioctls - when parsing nsswitch.conf(5).

Reviewed by: imp, markj
Approved by: re (gjb)
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17564