Fix error code if file isn't readable.

Approved by:	cperciva
MFC after:	1 week

usr.sbin/chkgrp/chkgrp.c

@@ -73,7 +73,7 @@ main(int argc, char *argv[])
 
     /* open group file */
     if ((gf = fopen(gfn, "r")) == NULL)
-	err(EX_IOERR, "%s", gfn); /* XXX - is IO_ERR the correct exit code? */
+	err(EX_NOINPUT, "%s", gfn);
 
     /* check line by line */
     while (++n) {