(bug 33037) Special:newfiles treat its subpage parameter as a limit

(bug 33037) Special:newfiles treat its subpage parameter as a limit

This restores the older behaviour of Special:newfiles. This allows
people to easily include variable sized galleries via
{{Special:newfiles/12}} syntax (people could of course previously do
{{special:newfiles|limit=12}} but less people know that.

Also makes IndexPager::setLimit validate the limit in the same way
it validates the limit coming from a url, and makes it so that
calling setLimit takes precedence over its built in determine the limit
code.

p.s. This is my first commit in our great glourious git future, so
let me know if i did anything wrong

Change-Id: I7fc148e56e43b7c453a1a9559ef74b8b3119c6fe