[REF] mail, various: rename subtype parameter of message_post to subtype_xmlid

PURPOSE

Clean posting process and improve mail.message definition and comprehension.

SPECIFICATIONS

In order to be more explicit subtype parameter is renamed to subtype_xmlid.
It therefore clearly indicates it should be a valid subtype Xml ID. Support
of ill formatted Xml IDs is removed because there is no reason to try to
add some random prefix. Give something that exists or go to hell, punk !

LINKS

Task ID 2071556
PR #38692