ADMIN: dyncookie: implement a simple dynamic cookie calculator

This utility can be useful to figure what cookie value a server will
have based on the secret, its IP and its port.
diff --git a/.gitignore b/.gitignore
index cd92620..ec9dd62 100644
--- a/.gitignore
+++ b/.gitignore
@@ -40,6 +40,7 @@
 *.bak
 # And reject some specific files
 /admin/halog/halog
+/admin/dyncookie/dyncookie
 /admin/iprange/ip6range
 /admin/iprange/iprange
 /admin/systemd/haproxy.service