It defaults to "-Q" but users can instead use "-q", which is useful if
"site-start.el" contains essential settings.  Also add `EMACS_BATCH'
and `EMACS_ORG', and rework use of related variables.

Re magit/magit#5472.

This counters Org commit 39264d3d41213085502dc0c2572f50855ea587a1.

- Files are not the only inputs to `texi', so we cannot rely on their
  modification times alone, to determine whether a rebuild is required.
  The new `redo-docs' always regenerates the texi file and thus all
  other formats.  Use that target before publishing the manual.

- Teach `html-dir' to only regenerate if the texi file has actually
  been touched.

- However when `html-dir' does regenerate, we must first remove all
  existing html files, to avoid files that used to be generated but
  are not being generated anymore, from sticking around.