[Bug-8370]

Provide a mechanism to specify the max_multipart_length of multipart data.
Remove MAX_MULTIPART_LENGTH constant and replace with a option to CGI.new,
that takes either a simple Integer scalar or a lambda that returns an Integer, for more
complex length limit calculation logic.
Modify multipart tests to be able to accept same options as CGI.new.
There is double "the" here.