schannel: improve handshake procedure

schannel: improve handshake procedure

- During handshake, do not require reading more data if unprocessed
  encrypted data that may be a complete TLS record is already available.

- During handshake, check that the socket is writeable before processing
  encrypted data that may require an immediate reply to the server.

These two fixes are for issues that were found during renegotiation
testing but could affect any handshake.

Prior to this change it was possible in some abnormal network conditions
for the Schannel TLS handshake procedure to erroneously wait or error.

Ref: https://github.com/curl/curl/pull/18125

Closes https://github.com/curl/curl/pull/18323