When we reach a whitespace after a section name, we assume that what
will follow will be a quoted subsection name.  Pass the current position
of the line being parsed to the subsection parser, so that it can
validate that subsequent characters are additional whitespace or a
single quote.

Previously we would begin parsing after the section name, looking for
the first quotation mark.  This allows invalid characters to embed
themselves between the end of the section name and the first quotation
mark, eg `[section foo "subsection"]`, which is illegal.