ACTION-534 7.4.2 replacement text

Old text for 7.4.2: has some problems in my mind, namely around executing
plugins. I propose the following changes:

old: Web user agents MUST NOT expose programming interfaces which permit
installation of software, or execution of privileged code without a user
intervention.
new: Web user agents MUST NOT expose programming interfaces which permit
installation of software, or execution of privileged code not previously
installed and registered with the user agent without a user intervention.

rationale: Someone could consider Acrobat Reader privileged code (it can do
all sorts of stuff), but clearly we don't want to prompt every time a user
navigates to a PDF.

I'm sure that at the face-to-face I had some other issues with this, but too
much time has passed and after re-reading the spec they aren't surfacing. So
I will live with the rest of the text.