Fix #2629. Add new config option `latex_keep_old_macro_names`

The cause of the issue was a clash about ``\titleref`` macro being already
defined in memoir class context. To avoid similar problems, this makes
the text styling macros

  ``\strong``, ``\code``, ``\bfcode``, ``\email``, ``\tablecontinued``,
  ``\titleref``, ``\menuselection``, ``\accelerator``, ``\crossref``,
  ``\termref``, ``\optional``,

also available with ``\sphinx`` prefix, with a conf.py boolean option to
let sphinx.sty only define ``\sphinx``-prefixed macros. As default value
is False, backwards compatibility is maintained.

On this occasion, some internal non-public macros have been renamed with
prefix ``\spx@``. The command

    find . -name '*.sty' -exec grep -l \\\\spx@ {} \;

has been executed in TeXLive 2015 and 2016 installations to check no
package defines macros starting with ``\spx@``.

Some internal macros having public names (because they are written by
latex.py into the body of the latex document) have been renamed to have
``\sphinx`` prefix. The macros in sphinx.sty starting with \py@, or \DU,
or \PYG have not been modified. Similarly ``\release``, ``\version``,
``\releasename``, etc... have not been renamed.

doc/config.rst

@@ -1565,6 +1565,21 @@ These options influence LaTeX output.
       value selected the ``'inline'`` display.  For backwards compatibility,
       ``True`` is still accepted.
 
+.. confval:: latex_keep_old_macro_names
+
+   If ``True`` (default) the ``\strong``, ``\code``, ``\bfcode``, ``\email``,
+   ``\tablecontinued``, ``\titleref``, ``\menuselection``, ``\accelerator``,
+   ``\crossref``, ``\termref``, and ``\optional`` text styling macros are
+   pre-defined by Sphinx and may be user-customized by some
+   ``\renewcommand``'s inserted either via ``'preamble'`` key or :dudir:`raw
+   <raw-data-pass-through>` directive. If ``False``, only ``\sphinxstrong``,
+   etc... macros are defined (and may be redefined by user). Setting to
+   ``False`` may help solve macro name conflicts caused by user-added latex
+   packages.
+
+   .. versionadded:: 1.4.5
+
+
 .. confval:: latex_elements
 
    .. versionadded:: 0.5