WinTLS: Fix PKCS12 loading

It appears that, contrary to most documentation, CERT_CONTEXT* is eaten
alive by AcquireCredentialsHandle(). Hence you cannot really reuse that.
AppleTLS: Enable --certificate

Tested against self-signed CA with nginx
WinTLS: Enable --certifcate support
Document WinTLS, AppleTLS, PKCS12 for --certificate
Document that AppleTLS, WinTLS does not support --ca-certitifcate