Bug fix serializing mixed lists containing single-line dt-dds.

* Wikitext ";a:\n:*b" was not being serialized correctly -- the
  ":" at the end of the first line was not being recognized as
  a single-line dt-dd.

  Test case extracted from en:Permian era

* This patch eliminates RT diffs from en:Permian era

* Unrelated cleanup:
  - Removed 'single_line_dtdd' flag from dd tokens and used
    the existing stx='row' flag that indicates the same thing.
  - Verified that fixes from 02c3d63 that added this flag are
    still intact after the cleanup.

* No change in parser tests (TODO: Add more parser tests)

Change-Id: I6efb137c6d706d164acf90a325d35235d7a05562