pfctl: Use pfctl_fopen

Use pfctl_fopen (which checks to ensure the given file isn't a directory)
for pfctl -f /path/to/pf.conf. Otherwise, if you accidentally use a dir
instead of a file (e.g. if you have an /etc/pf directory and don't notice
the name when tab-completing), you successfully install an empty ruleset.
ok sashan@

Obtained from:	OpenBSD, sthen <[email protected]>, 2f48098846
Sponsored by:	Rubicon Communications, LLC ("Netgate")

Author: kprovost

sbin/pfctl/parse.y

@@ -7091,7 +7091,7 @@ pushfile(const char *name, int secret)
 			free(nfile);
 			return (NULL);
 		}
-	} else if ((nfile->stream = fopen(nfile->name, "r")) == NULL) {
+	} else if ((nfile->stream = pfctl_fopen(nfile->name, "r")) == NULL) {
 		warn("%s: %s", __func__, nfile->name);
 		free(nfile->name);
 		free(nfile);