[MINOR] Allow to specify a domain for a cookie

This patch allows to specify a domain used when inserting a cookie
providing a session stickiness. Usefull for example with wildcard domains.

The patch adds one new variable to the struct proxy: cookiedomain.
When set the domain is appended to a Set-Cookie header.

Domain name is validated using the new invalid_domainchar() function.
It is basically invalid_char() limited to [A-Za-z0-9_.-]. Yes, the test
is too trivial and does not cover all wrong situations, but the main
purpose is to detect most common mistakes, not intentional abuses.

The underscore ("_") character is not RFC-valid but as it is
often (mis)used so I decided to allow it.
diff --git a/src/proto_http.c b/src/proto_http.c
index 2c07030..3caeba1 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -3222,6 +3222,9 @@
 				      t->be->cookie_name,
 				      t->srv->cookie ? t->srv->cookie : "; Expires=Thu, 01-Jan-1970 00:00:01 GMT");
 
+			if (t->be->cookiedomain)
+				len += sprintf(trash+len, "; domain=%s", t->be->cookiedomain);
+
 			if (unlikely(http_header_add_tail2(rep, &txn->rsp, &txn->hdr_idx,
 							   trash, len)) < 0)
 				goto return_bad_resp;