BUG/MINOR: unix: properly check for octal digits in the "mode" argument

A config containing "stats socket /path/to/socket mode admin" used to
silently start and be unusable (mode 0, level user) because the "mode"
parser doesn't take care of non-digits. Now it properly reports :

   [ALERT] 276/144303 (7019) : parsing [ext-check.cfg:4] : 'stats socket' : ''mode' : missing or invalid mode 'admin' (octal integer expected)'

This can probably be backported to 1.7, 1.6 and 1.5, though reporting
parsing errors in very old versions probably isn't a good idea if the
feature was left unused for years.
diff --git a/src/proto_uxst.c b/src/proto_uxst.c
index 80237b1..80ba8cc 100644
--- a/src/proto_uxst.c
+++ b/src/proto_uxst.c
@@ -623,12 +623,15 @@
 /* parse the "mode" bind keyword */
 static int bind_parse_mode(char **args, int cur_arg, struct proxy *px, struct bind_conf *conf, char **err)
 {
-	if (!*args[cur_arg + 1]) {
-		memprintf(err, "'%s' : missing mode (octal integer expected)", args[cur_arg]);
+	char *endptr;
+
+	conf->ux.mode = strtol(args[cur_arg + 1], &endptr, 8);
+
+	if (!*args[cur_arg + 1] || *endptr) {
+		memprintf(err, "'%s' : missing or invalid mode '%s' (octal integer expected)", args[cur_arg], args[cur_arg + 1]);
 		return ERR_ALERT | ERR_FATAL;
 	}
 
-	conf->ux.mode = strtol(args[cur_arg + 1], NULL, 8);
 	return 0;
 }