login: move check_securetty to libbb

Signed-off-by: Kaarle Ritvanen <kaarle.ritvanen@datakunkku.fi>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/securetty.c

@@ -0,0 +1,22 @@
+/* vi: set sw=4 ts=4: */
+/*
+ * /etc/securetty checking.
+ *
+ * Licensed under GPLv2, see file LICENSE in this source tree.
+ */
+#include "libbb.h"
+
+int FAST_FUNC check_securetty(const char *short_tty)
+{
+	char *buf = (char*)"/etc/securetty"; /* any non-NULL is ok */
+	parser_t *parser = config_open2("/etc/securetty", fopen_for_read);
+	while (config_read(parser, &buf, 1, 1, "# \t", PARSE_NORMAL)) {
+		if (strcmp(buf, short_tty) == 0)
+			break;
+		buf = NULL;
+	}
+	config_close(parser);
+	/* buf != NULL here if config file was not found, empty
+	 * or line was found which equals short_tty */
+	return buf != NULL;
+}