schannel: Added special shutdown check for Windows 2000 Professional

Windows 2000 Professional:  Schannel returns SEC_E_OK instead
of SEC_I_CONTEXT_EXPIRED. If the length of the output buffer
is zero and the first byte of the encrypted packet is 0x15,
the application can safely assume that the message was a
close_notify message and change the return value to
SEC_I_CONTEXT_EXPIRED.

Connection shutdown does not mean that there is no data to read
Correctly handle incomplete message and ask curl to re-read
Fixed buffer for decrypted being to small
Re-structured read condition to be more effective
Removed obsolete verbose messages
Changed memory reduction method to keep a minimum buffer of size 4096