Implement acceptance policies in SoupCookieJar
Through the "accept-policy" property in the jar we can now set one
among three predetermined policies: accept all cookies
(SOUP_COOKIE_JAR_ACCEPT_ALWAYS), accept none
(SOUP_COOKIE_JAR_ACCEPT_NEVER) and only accept cookies set by the main
document we are loading (SOUP_COOKIE_JAR_ACCEPT_NO_THIRD_PARTY).
A new property, "first-party", is added to SoupMessage so that the
application using libsoup can inform us of what is the main document
URI for each message that is requested.