Deprecation preferences

There are quite a few places in Kernel that reference Preferences
showDeprecationWarnings.

I'd like to turn this preference into a new-world pragma based
preference, and Deprecation (despite being an exception) seems like a
good place to hang the preference.

So instead of saying "Preferences showDeprecationWarnings" you'd say
"Deprecation showWarnings", and the preference would be called "Show
deprecation warnings".

Any strong objection?

frank