ldap: support insecure mode for Windows native LDAP
- Bypass cert verification if verifypeer is disabled.
Prior to this change libcurl lacked the ability to bypass certificate
verification for Windows native LDAP (USE_WIN32_LDAP). A comment said
"Win32 LDAP SDK does not support insecure mode without CA!" but I found
that we can bypass the check by setting a verify callback to override
Windows' internal verify check.