SSL_CIPHER lookup functions.

Add tables to convert between SSL_CIPHER fields and indices for ciphers
and MACs.

Reorganise ssl_ciph.c to use tables to lookup values and load them.

New functions SSL_CIPHER_get_cipher_nid and SSL_CIPHER_get_digest_nid.

Add documentation.

Reviewed-by: Rich Salz <rsalz@openssl.org>