Add OpenUpgrade specific functions for message_last_post fields

Add set_message_last_post() which fills message_last_post values

Signed-off-by: Sandy Carter <sandy.carter@savoirfairelinux.com>
Add documentation of OpenUpgrade 8.0 specific API

Signed-off-by: Sandy Carter <sandy.carter@savoirfairelinux.com>
Merge pull request #13 from savoirfairelinux/message_last_post

[ADD] Add OpenUpgrade specific functions for message_last_post fields