Allow pow-params to appear multiple times.

Allow pow-params to appear multiple times.

Relatedly, we only require that a pow-params line has _1_ argument
(since future versions might have nothing but a scheme).

See torspec#272 for discussion;
also see proposal 356 for why the fingerprinting opportunity here
isn't a big deal.

We probably shouldn't merge this until torspec!390 is in,
just in case we decide _not_ to take this approach.

(I haven't reindented some code here yet, to make the diff easier to read.)