Don't resume OpenSSL DTLS session for PSK-NEGOTIATE

Don't resume OpenSSL DTLS session for PSK-NEGOTIATE

Now that we are using a custom extension instead of the session-id
hack, we no longer need to pretend to resume a session. It was causing
a session-id of 32 zeroes to be included in the ClientHello. With
OpenSSL 1.1+, that was causing fragmentation which ocserv couldn't
cope with.

Perhaps ocserv *should* have coped with that fragmentation, and perhaps
we should increase our initial idea of the MTU to avoid the fragmentation.
But certainly we shouldn't be including an all-zero session-id for
resumption either.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>