Handle quotation marks and apostrophes in ‘sgml-quote’

Handle quotation marks and apostrophes in ‘sgml-quote’

To be able to use text in an HTML argument, quotation marks need
to be replaced with an appropriate character reference.  Make
‘sgml-quote’ do that.

While at it, fix entiteis not being unquoted if they lack closing
semicolon (e.g. ‘&amp’) occuring at the very end of a region.
Even though unlikely, make ‘sgml-quote’ handle this scenario.

* lisp/textmodes/sgml-mode.el (sgml-quote): Handle quotation marks and
apostrophes.  Match entities lacking semicolon at the end of regions.
* test/lisp/textmodes/sgml-mode-tests.el (sgml-quote-works): New test
case for ‘sgml-quote’ function.