schannel: schannel_recv overhaul

This commit is several drafts squashed together. The changes from each
draft are noted below. If any changes are similar and possibly
contradictory the change in the latest draft takes precedence.

Bug: https://github.com/bagder/curl/issues/244
Reported-by: Chris Araman

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%% Draft 1
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- return 0 if len == 0. that will have to be documented.
- continue on and process the caches regardless of raw recv
- if decrypted data will be returned then set the error code to CURLE_OK
and return its count
- if decrypted data will not be returned and the connection has closed
(eg nread == 0) then return 0 and CURLE_OK
- if decrypted data will not be returned and the connection *hasn't*
closed then set the error code to CURLE_AGAIN --only if an error code
isn't already set-- and return -1
- narrow the Win2k workaround to only Win2k

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%% Draft 2
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- Trying out a change in flow to handle corner cases.

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%% Draft 3
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- Back out the lazier decryption change made in draft2.

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%% Draft 4
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- Some formatting and branching changes
- Decrypt all encrypted cached data when len == 0
- Save connection closed state
- Change special Win2k check to use connection closed state

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%% Draft 5
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- Default to CURLE_AGAIN in cleanup if an error code wasn't set and the
connection isn't closed.

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%% Draft 6
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- Save the last error only if it is an unrecoverable error.

Prior to this I saved the last error state in all cases; unfortunately
the logic to cover that in all cases would lead to some muddle and I'm
concerned that could then lead to a bug in the future so I've replaced
it by only recording an unrecoverable error and that state will persist.

- Do not recurse on renegotiation.

Instead we'll continue on to process any trailing encrypted data
received during the renegotiation only.

- Move the err checks in cleanup after the check for decrypted data.

In either case decrypted data is always returned but I think it's easier
to understand when those err checks come after the decrypted data check.

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%% Draft 7
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- Regardless of len value go directly to cleanup if there is an
unrecoverable error or a close_notify was already received. Prior to
this change we only acknowledged those two states if len != 0.

- Fix a bug in connection closed behavior: Set the error state in the
cleanup, because we don't know for sure it's an error until that time.

- (Related to above) In the case the connection is closed go "greedy"
with the decryption to make sure all remaining encrypted data has been
decrypted even if it is not needed at that time by the caller. This is
necessary because we can only tell if the connection closed gracefully
(close_notify) once all encrypted data has been decrypted.

- Do not renegotiate when an unrecoverable error is pending.

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%% Draft 8
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- Don't show 'server closed the connection' info message twice.

- Show an info message if server closed abruptly (missing close_notify).