Backport quote-related commits from Emacs

Backport quote-related commits from Emacs

These changes are the combination of the commits below (in order
applied).  Unlike other backports, these are applied in one commit
because they are closely related and because some of the later commits
reverse changes in earlier commits.  Changes to message and help quote
display for Emacs 25 seem to be here to stay.  The quote protections are
required to ensure grave accents and apostrophes are displayed as
intended in Emacs 25, and they should not affect earlier versions.

  Quoting fixes in lisp/org
  6cd2629bd1b3ce77d29ac28e29237c94a4197116
  Paul Eggert
  Mon Aug 31 00:29:40 2015 -0700

  More-conservative ‘format’ quote restyling
  fbb5531fa11d13854b274d28ccd329c9b6652cfc
  Paul Eggert
  Sun Aug 23 22:39:06 2015 -0700

  Prefer directed to neutral quotes
  875a5d0ead827d3da32ecbd30e739a29f07bbc87
  Paul Eggert
  Mon Aug 24 23:57:25 2015 -0700

  Escape ` and ' in doc
  19532d147b431a4fe34f088d6de07891c48e2c5c
  Paul Eggert
  Tue Sep 1 18:23:21 2015 -0700

  Fix some more docstring etc. quoting problems
  26bd978d87dfbf9baa115cd961a67d42b416c4bf
  Paul Eggert
  Thu Sep 3 15:32:54 2015 -0700

  Go back to grave quoting in source-code docstrings etc.
  6afef3f6ca2f3009c722b84e249903b7f807b044
  Paul Eggert
  Mon Sep 7 16:09:27 2015 -0700