Remove arbitrary limit SSH2_MKKEY_ITERS.

Remove arbitrary limit SSH2_MKKEY_ITERS.

Tim Kosse points out that we now support some combinations of crypto
primitives which break the hardwired assumption that two blocks of
hash output from the session-key derivation algorithm are sufficient
to key every cipher and MAC in the system.

So now ssh2_mkkey is given the desired key length, and performs as
many iterations as necessary.