rsa: big exponents are exponents, too!

The key validity check barfs on exponents larger than 2^16+1.  However, there's no real reason why exponents have to be smaller than that -- those are perfectly legal exponents, and Scallion generates keys that have quite large exponents indeed!  (For instance, my key, 0x9080C092EA80E0B4, has exponent 0xE8BD99D9.)

This pull request removes that restriction.

(Verified: passed 'make test'.)