Improve text writing in help output

Improve text writing in help output

- Use new facilities in TextWriter for ensuring consistent line breaks
  in the output, removing the need to carefully track where empty lines
  are written out to produce valid reStructuredText and to not produce
  too many empty lines in console output.  Replace most explicit
  newlines with ensureEmptyLine() and friends.
- Use TextWriter consistently in the help writing, making the caller
  responsible of creating the single writer used for the output.  This
  makes all the output share the same writer (and thus the same state),
  and removes the need to access the underlying stream for the writer.

Change-Id: I9d6a54dbb1dbb64fbd6ac95667d6ed9988288a60