schannel: Make CURLOPT_CAINFO work better on Windows 7

- Support hostname verification via alternative names (SAN) in the
  peer certificate when CURLOPT_CAINFO is used in Windows 7 and earlier.

CERT_NAME_SEARCH_ALL_NAMES_FLAG doesn't exist before Windows 8. As a
result CertGetNameString doesn't quite work on those versions of
Windows. This change provides an alternative solution for
CertGetNameString by iterating through CERT_ALT_NAME_INFO for earlier
versions of Windows.

Prior to this change many certificates failed the hostname validation
when CURLOPT_CAINFO was used in Windows 7 and earlier. Most certificates
now represent multiple hostnames and rely on the alternative names field
exclusively to represent their hostnames.

Reported-by: Jeroen Ooms

Fixes https://github.com/curl/curl/issues/3711
Closes https://github.com/curl/curl/pull/4761

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