Go back to internal OCB implementation

After further discussion, the Mosh maintainers have decided to stick
with the internal OCB implementation for this release. Restore support
for using OpenSSL’s AES but internal OCB. To make this commit easy to
audit, restore the code exactly, including calls to AES functions that
are deprecated in OpenSSL 3; a future commit will update ocb_internal.cc
to use EVP instead of directly calling the AES primitives.

In anticipation of future changes, preserve support for OpenSSL’s
AES-OCB, but don’t compile it in. Add
--with-crypto-library=openssl-with-openssl-ocb and
--with-crypto-library=openssl-with-internal-ocb options to configure so
that developers can easily test Mosh using OpenSSL’s AES-OCB. These
options are intended only for testing, are undocumented, and are not
subject to any API stability guarantees.

Rework configure to look for all possible cryptography libraries first
and then dispatch on --with-crypto-library as appropriate.