org.el: Emulate outline's buffer-invisiblity-spec in orgstruct-mode

* org.el (orgstruct-mode): Emulate outline's
buffer-invisiblity-spec.
org.el: Use most general key bindings in orgstruct-mode

* org.el (orgstruct-setup): Generalize key bindings.
Fix `org-export-backends' value

* lisp/org.el (org-export-backends): Remove duplicates.  Reorder
  alphabetically.
org.el: Make the hijacker commands of orgstruct-mode try both general and specialised (function) key bindings

* org.el (orgstruct-make-binding): Make the hijacker commands
try both general and specialized (function) key bindings.
Fix broken org-bullets.

* contrib/lisp/org-bullets.el (org-bullets-mode): Remove bad require.
  Add comma in backticked list to evaluate org-outline-regexp-bol,
  which replaced a regexp constant.
ox-html.el: Cleanup pre/postamble handling

* lisp/ox-html.el (org-html--timestamp-format): New variable used
in the metadata and the pre/postamble.
(org-html-style-default): Make the preamble and postamble use the
same style.  Make all anchors font-size %100.  Remove margin from
the content section.
(org-html-container-element): Fix docstring.
(org-html-postamble-format, org-html-preamble-format): Update
docstrings.
(org-html-template): Use `org-html--build-pre/postamble'.
(org-html--build-pre/postamble): New function, combining the
pre/postamble generator.  Merge lists of formatters from the
preamble and postamble.  Fix bug, using :time-stamp-file instead
of :with-date for auto display of date: this brings usage in-line
with the latex and beamer exporter.
(org-html--build-postamble, org-html--build-postamble): Delete.