schannel: Implement new buffer size strategy
Increase decrypted and encrypted cache buffers using limitted
doubling strategy. More information on the mailinglist:
http://curl.haxx.se/mail/lib-2012-06/0255.html
It updates the two remaining reallocations that have already been there
and fixes the other one to use the same "do we need to increase the
buffer"-condition as the other two. CURL_SCHANNEL_BUFFER_STEP_SIZE was
renamed to CURL_SCHANNEL_BUFFER_FREE_SIZE since that is actually what it
is now. Since we don't know how much more data we are going to read
during the handshake, CURL_SCHANNEL_BUFFER_FREE_SIZE is used as the
minimum free space required in the buffer for the next operation.
CURL_SCHANNEL_BUFFER_STEP_SIZE was used for that before, too, but since
we don't have a step size now, the define was renamed.