TLSv1.3 behaves differently in how PSK identity/PSK identity hints are
exchanged, at least in regards to OpenSSL. This caused the TLS client to
not send their TLS identity to the server, which rejected the connection
(it expected "luksrku v1"). Couldn't solve it with TLSv1.3, so we're now
simply forcing TLSv1.2.
While the PSK cipher suites do not use any ECDHE/RSA signatures, in the
future someone may change the code. In that case, as a robustness
measure, already set the acceptable signature algorithms now.
Additionally upgrade to OpenSSL v1.1.0e and include the comment to
include X448 once it becomes available for TLS ECDHE (it's not yet,
unfortunately).
