Include a TLS record header when switching to the legacy stack.

When switching to the legacy TLS stack we previously copied any remaining
handshake messages into the receive buffer, but do not include any TLS
record header (largely due to the fact that we've already processed part
of the TLS record that we actually received - that part is placed into the
init_buf). This worked fine with the old record layer implementation,
however the new record layer expects to find the TLS record header.

This means that if we switch from the new stack to the legacy stack (i.e.
the remote side does not support TLSv1.3) and there is more than one
handshake message in the TLS plaintext record (which Microsoft's TLS
stack is known to do), we now read a TLS record of zero bytes instead of
getting the correct length.

Fix this by generating a pseudo-TLS record header when switching from the
new TLS stack to the legacy stack.

Found the hard way by guenther@.

Thanks to tb@ for coming up with a reproducible test case and doing much
of the debugging.

ok inoguchi@ tb@

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