fix several problems with config parser:

 a bug where it underflows the string
 a bug where it never frees parser_t struct
make read_config() return 0 if parser is NULL,
 make config_close() accept and ignore NULL parser -
 eliminates many if() blocks
reverse the sense of parser bit flags - negative flags
 are harder to grok.
hexdump: revert the change to use config parser, it is BIGGER
 and also requires additional quirks in parser
*: explicitly use PARSER_NORMAL instead of 0

function                                             old     new   delta
login_main                                          1575    1596     +21
config_close                                          18      29     +11
bbunpack                                             383     391      +8
qgravechar                                           106     109      +3
rtnl_tab_initialize                                  121     117      -4
expand                                              1697    1693      -4
man_main                                             717     712      -5
nameif_main                                          674     668      -6
hexdump_main                                         597     591      -6
read_config                                          217     209      -8
dnsd_main                                           1478    1470      -8
sysctl_main                                          203     189     -14
config_open2                                          44      25     -19
make_device                                         1177    1141     -36
config_read                                          597     549     -48
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 4/11 up/down: 43/-158)         Total: -115 bytes

1 files changed, 9 insertions, 12 deletions

diff --git a/loginutils/login.c b/loginutils/login.c
index 5a75ed218..8732b99f1 100644
--- a/loginutils/login.c
+++ b/loginutils/login.c
@@ -139,20 +139,17 @@ static ALWAYS_INLINE void die_if_nologin(void) {}
 #if ENABLE_FEATURE_SECURETTY && !ENABLE_PAM
 static int check_securetty(void)
 {
-	char *buf;
-	int ret = 1;
+	char *buf = (char*)"/etc/securetty"; /* any non-NULL is ok */
 	parser_t *parser = config_open2("/etc/securetty", fopen_for_read);
-	/* N.B. A missing securetty file is not an error. */
-	if (parser) {
-		while (config_read(parser, &buf, 1, 1, "# \t", 0)) {
-			if (strcmp(buf, short_tty) == 0)
-				break;
-		}
-		config_close(parser);
-		// buf != NULL here iff config file was empty (OK) or buf equals short_tty (OK)
-		ret = buf != NULL;
+	while (config_read(parser, &buf, 1, 1, "# \t", PARSE_NORMAL)) {
+		if (strcmp(buf, short_tty) == 0)
+			break;
+		buf = NULL;
 	}
-	return ret;
+	config_close(parser);
+	/* buf != NULL here if config file was not found, empty
+	 * or line was found which equals short_tty */
+	return buf != NULL;
 }
 #else
 static ALWAYS_INLINE int check_securetty(void) { return 1; }