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

lib/libc/net/nslexer.l

@@ -50,6 +50,7 @@ static char *rcsid =
 
 %}
 
+%option never-interactive
 %option noinput
 %option nounput
 %option yylineno