[CLEANUP] add a few "const char *" where appropriate

As suggested by Markus Elfring, a few "const char *" have replaced
some "char *" declarations where a function is not expected to
modify a value. It does not change the code but it helps detecting
coding errors.
diff --git a/src/queue.c b/src/queue.c
index 3528537..8b37da0 100644
--- a/src/queue.c
+++ b/src/queue.c
@@ -26,7 +26,7 @@
 /* returns the effective dynamic maxconn for a server, considering the minconn
  * and the proxy's usage relative to its saturation.
  */
-unsigned int srv_dynamic_maxconn(struct server *s)
+unsigned int srv_dynamic_maxconn(const struct server *s)
 {
 	return s->minconn ? 
 		((s->maxconn * s->proxy->nbconn / s->proxy->maxconn) < s->minconn) ? s->minconn :