QUIC: better approach for premature handshake completion.

QUIC: better approach for premature handshake completion.

Using SSL_in_init() to inspect a handshake state was replaced with
SSL_is_init_finished().  This represents a more complete fix to the
BoringSSL issue addressed in 22671b37e.

This provides awareness of the early data handshake state when using
OpenSSL 3.5 TLS callbacks in 0-RTT enabled configurations, which, in
particular, is used to avoid premature completion of the initial TLS
handshake, before required client handshake messages are received.

This is a non-functional change when using BoringSSL.  It supersedes
testing non-positive SSL_do_handshake() results in all supported SSL
libraries, hence simplified.

In preparation for using OpenSSL 3.5 TLS callbacks.