Instead of using a fixed 256 byte buffer in the connectdata struct.
In my build, this reduces the size of the connectdata struct by 11.8%,
from 2160 to 1904 bytes with no functionality or performance loss.
This also fixes a bug in schannel's Curl_verify_certificate where it
called Curl_sspi_strerror when it should have called Curl_strerror for
string from GetLastError. the only effect would have been no text or the
wrong text being shown for the error.
Co-authored-by: Jay Satiro
Closes#3612
