Rename ap_func_attr_* macros to AP_FN_ATTR_* Add macro for attribute alloc_size on newer gcc's

Avoid "`sentinel' attribute directive ignored" warning with gcc 3.x

Add __attribute__((sentinel)) to a few functions that require a terminal NULL argument.

Tweak some doxygen comments to get these functions to show up in the right places in the generated doc.

mod_session_cookie, mod_session_dbd: Make sure cookies are set both within the output headers and error output headers, so that the session is maintained across redirects.

mod_session_cookie: Make sure that cookie attributes are correctly included in the blank cookie when cookies are removed. This fixes an inability to log out when using mod_auth_form.

mod_session_cookie: Add a session implementation capable of storing session information within cookies on the browser. Useful for high volume sites where server bound sessions are too resource intensive.