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.