Fix problem with links at beginning of buf and paragraphs.

Fix problem with links at beginning of buf and paragraphs.

* experimental/muse-xml.el (muse-xml-markup-regexps): Update to do what I
  mean.

* muse-docbook.el (muse-docbook-markup-regexps): Ditto.

* muse-html.el (muse-html-markup-regexps): Ditto.

* muse-publish.el (muse-publishing-last-position): New variable that
  indicates our last buffer position while publishing the current rule.
  Used to make sure the process doesn't get stalled.
  (muse-publish-markup): Fix edge case where links at beginning of
  buffer, with read-only text, would prevent the beginning of buffer from
  being matched.
  (muse-publish-markup-directive): Fix case where two directives of the
  same size would prevent any following directives from being
  interpreted.  We accomplish this by setting the last published position
  to nil after each directive.
git-archimport-id: mwolson@gnu.org--2005/muse--main--1.0--patch-208