Disable the embedded syntax feature of newer syntax/rst.vim:

- It doesn't work with both "c" and "cpp" active simultaneously, since both
  rely on including syntax/c.vim, and the double inclusion of this file
  causes problems.

- It requires a fairly new Vim, and we'd like to support older ones, too.

- It marks the block with NoSpell, which we don't want.

- It's easier to disable the support in syntax/rst.vim entirely than to
  partially use it and work around its limitations.

Also:

- Define variable g:rstEmbeddedLangs to allow users to control which
  languages are embedded.
Adjust embedded languages in .rst files.

- Use syntax/rst.vim's region names so highlighting is consistent
  for both highlighted and unhighlighted language in code-blocks.

- Refactor function to embed languages to allow non-redundant
  implementation for code-blocks with unhighlighted languages.

- Warn about use of g:rst_syntax_code_list to avoid user surprise.