Allow API meta=siteinfo to return list of know variable IDs

Allow API meta=siteinfo to return list of know variable IDs

To know whether something like {{foo}} is a template, variable, or
function, you have to first check if it's a magic word, and then look up
the magic word ID in the list of variables and the list of function
hooks. The API already can return the list of known magic word mappings
(siprop=magicwords) and function hooks (siprop=functionhooks), this
completes the set by allowing siprop=variables.

Change-Id: If13df9c3796ea6815393d847a3c6f70380bba58a