[MINOR] config: allow "slowstart 0s"

Sometimes it's useful to be able to disable slowstart by setting "slowstart 0".
diff --git a/src/cfgparse.c b/src/cfgparse.c
index f21c278..bddb6e7 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -3305,7 +3305,7 @@
 					err_code |= ERR_ALERT | ERR_FATAL;
 					goto out;
 				}
-				if (val <= 0) {
+				if (val < 0) {
 					Alert("parsing [%s:%d]: invalid value %d for argument '%s' of server %s.\n",
 					      file, linenum, val, args[cur_arg], newsrv->id);
 					err_code |= ERR_ALERT | ERR_FATAL;