[ticket/11769] Correctly supply the post author's username in posting.php

Only supply the username, when it is a guest posting or we edit and it was
supplied, otherwise post_data might hold data of the post we quote, in which
case username is the original poster, not the current one.

PHPBB3-11769
[ticket/11769] Fix language issues in the doc blocks

PHPBB3-11769
[ticket/11769] Allow '0' as username in notification mails

PHPBB3-11769
[ticket/11769] Allow '0' as username

PHPBB3-11769
[ticket/11769] Allow using 0 as poster name

PHPBB3-11769
Merge remote-tracking branch 'nickvergessen/ticket/11769' into prep-release-3.0.12

* nickvergessen/ticket/11769:
  [ticket/11769] Allow using 0 as poster name
  [ticket/11769] Allow '0' as username
  [ticket/11769] Allow '0' as username in notification mails
  [ticket/11769] Fix language issues in the doc blocks
  [ticket/11769] Correctly supply the post author's username in posting.php