Patch 5554 by Goba (slightly modified):

  - Modifies _user_mail_text() to automatically override the default
    strings when needed and apply t() on the strings. This lets Drupal translators
    include default messages in translations, and also removes a lot of repeated code.
    Modified the internal $messageid of the approval message from
    welcome_approval_body/subject to approval_body/subject, so automatic variable
    name computation can be done. This does not affect any variable table keys, since
    the change was especially done to keep the variable names...

  - Adds %login_uri and %edit_uri to move away from the concatenation approach used in
    the default messages, thus making those messages visible for the .po extractor

  - Uses direct return in help text generation when no more action need to be taken
    on the help string (the '$output .=' approach was just plain silly looking and
    misleading, since there was no preceding or appended string in $output)