ENGINE_TABLE-based stuff - as described in crypto/engine/README.

Associated miscellaneous changes;
 - the previous cipher/digest hooks that hardwired directly to EVP's
   OBJ_NAME-based storage have been backed out. New cipher/digest support
   has been constructed and will be committed shortly.
 - each implementation defines its own ENGINE_load_<name> function now.
 - the "openssl" ENGINE isn't needed or loaded any more.
 - core (not algorithm or class specific) ENGINE code has been split into
   multiple files to increase readability and decrease linker bloat.
 - ENGINE_cpy() has been removed as it wasn't really a good idea in the
   first place and now, because of registration issues, can't be
   meaningfully defined any more.
 - BN_MOD_EXP[_CRT] support is removed as per the README.
 - a bug in enginetest.c has been fixed.

NB: This commit almost certainly breaks compilation until subsequent
changes are committed.