Separate OpenSSL-based OCB implementation from others

Split src/crypto/ocb.cc into two files – one containing the AES-OCB
implementation backed by OpenSSL, and the other containing
implementations backed by Apple Common Crypto and Nettle. This paves the
way for a new OpenSSL implementation that uses OpenSSL 1.1’s OCB support
directly, rather than one that merely uses OpenSSL to provide the
underlying block cipher.

Remove support for rijndael-alg-fst.c and compiler-provided AES
intrinsics, since they’re not in use anymore. (Mosh can still use
hardware-accelerated AES if it’s available; it just now relies
exclusively on the underlying cryptography library to accelerate AES if
possible.)

Update the build system to conditionally compile in either
ocb_openssl.cc or ocb_internal.cc, depending on which cryptography
library you pass to ./configure.

To make this commit easy to audit, ocb_openssl.cc and ocb_internal.cc
are trivially diffable against ocb.cc (now deleted). Expected diffs
consist of a copyright notice update, a preprocessor check to ensure the
appropriate cryptography implementation has been selected, and deletions
to remove code that’s no longer in use. This does mean a substantial
amount of code is duplicated between ocb_openssl.cc and ocb_internal.cc;
however, ocb_openssl.cc should be completely replaced soon, so it won’t
be an issue in the long term.

Bug: https://github.com/mobile-shell/mosh/issues/1174

configure.ac

@@ -399,6 +399,10 @@ case "${with_crypto_library}" in
     ;;
 esac
 
+AM_CONDITIONAL([CRYPTO_LIBRARY_OPENSSL], [test x$with_crypto_library = xopenssl])
+AM_CONDITIONAL([CRYPTO_LIBRARY_NETTLE], [test x$with_crypto_library = xnettle])
+AM_CONDITIONAL([CRYPTO_LIBRARY_APPLE], [test x$with_crypto_library = xapple-common-crypto])
+
 AC_ARG_ENABLE([static-crypto],
   [AS_HELP_STRING([--enable-static-crypto], [Link crypto library statically @<:@no@:>@])],
   [], [enable_static_crypto="$enable_static_libraries"])