bug#14003: 24.3.50; sgml-mode: support angle brackets inside jsp tags

[Dmitry Gutov]

sgml-mode has some support for preprocessing tags.
When it sees tags with percent characters inside (<% hello! %>), it says
that the tag's type is `jsp'.

The templating engine may be different (e.g. ERB or EJS), but all of
these examples can have arbitrary code inside, including angle brackets,
usually serving as comparison operators.

Before revno:111900, only having "<" inside a `jsp' tag was a problem:
it broke indentation on the following lines, until a closing containing
tag. With the aforementioned revision, having either "<" or ">" inside a
`jsp' tag breaks string highlighting, because they changes the depth in
parens returned by (syntax-ppss).

So, the previously discussed general problem
(http://lists.gnu.org/archive/html/emacs-devel/2012-06/msg00238.html)
got worse in practice.

I think we can make it better in this specific case without solving it
in general, and doing it for html-mode would be a big win.

Roughly, I propose to add a rule in sgml-syntax-propertize-function that
would forcibly mark all angle brackets between <% and %> as punctuation
syntax. Testing it with

(with-silent-modifications
  (put-text-property (point) (1+ (point))
                     'syntax-table (string-to-syntax ".")))

seems to help both with highlighting and indentation.

Before I started writing this text, I had another approach in mind, but
this is something that I can probably do in mmm-mode instead, without
changing sgml-mode, by wrapping submodes' syntax-propertize-functions.
Still, doing it sgml-mode should be simpler.

Thoughts?