Reduce code duplication in SSLv2 handshake handling

[baldwinmatt]

Problem:

Code duplication leads to bugs when one place is updated and another overlooked.

Proposed Solution:

Refactor such that the state machine error checking is common to both the s2n_negotiate and s2n_handshake_handle_sslv2 functions.

[lrstewart]

Not just the error checking: we also duplicate cleaning up the input stuffers, advancing the message, and maybe updating the hashes? We need to identify exactly what needs to be different about sslv2 and make that very clear from the code.

[jmayclin]

As of 2025-02-13, we still believe this is an issue. Marking this as medium size because it will likely touch the saw proofs.