SSL: support AES256 encryption of tickets.

SSL: support AES256 encryption of tickets.

This implies ticket key size of 80 bytes instead of previously used 48,
as both HMAC and AES keys are 32 bytes now.  When an old 48-byte ticket key
is provided, we fall back to using backward-compatible AES128 encryption.

OpenSSL switched to using AES256 in 1.1.0, and we are providing equivalent
security.  While here, order of HMAC and AES keys was reverted to make
the implementation compatible with keys used by OpenSSL with
SSL_CTX_set_tlsext_ticket_keys().

Prodded by Christian Klinger.