Make rest2html program run more easily

As interpreted by a Python 2.7 interpreter on a GNU system.

This shebang form is unusual (env python2 -S), nor is relying on
whitespace between shebang entries considered portable, de-facto nor
de-jure.

In addition, reloading sys seems to be necessary to allow for
setdefaultencoding.  That's apparently to prevent people from using
that symbol, but as it was working before somehow, just fix it.
Interpret titles and subtitles in rst

This hopes to make subtitles usable without changing the markup and
table-of-contents generation of documents much at all.

Previously, titles and subtitles[0] would introduce a lot of nesting
in a generated table of contents.  Enable both doctitle_xform and
sectsubtitle_xform to remove those.

However, merely enabling those will cause the existing top-level,
non-title HTML header sizes to be promoted to <h1>, since titles and
subtitles are not considered "sections", changing how the HTML output
looks a rather lot.

That's not how some sampled rsts (without subtitles) on work today:
the title is biggest at <h1>, and all other sections are <h2> or
below.  So set the initial_header_level to "2", so that the largest
text sections can have is <h2>, tied in size for the subtitle.

[0]: http://docutils.sourceforge.net/docs/user/rst/quickstart.html#document-title-subtitle