schannel: fix renegotiation

schannel: fix renegotiation

- Move the schannel_recv renegotiation code to function
  schannel_recv_renegotiate.

- Save the state of a pending renegotiation.

- Pre-empt schannel_recv and schannel_send to continue a pending
  renegotation.

- Partially block during renegotiation if necessary.

Prior to this change, since a1850ad7 (precedes 8.13.0), schannel_recv
did not properly complete renegotiation before attempting to decrypt
data. In some cases that could cause an error SEC_E_CONTEXT_EXPIRED.
Most of the time though DecryptMessage would succeed by chance and
return SEC_I_RENEGOTIATE which allowed the renegotiation to continue.

Reported-by: stephannn@users.noreply.github.com
Reported-by: Dustin L. Howett
Fixes https://github.com/curl/curl/issues/18029
Closes https://github.com/curl/curl/pull/18125