improve docu of SSL_CTX_use_PrivateKey()

SSL_CTX_use_RSAPrivateKey() adds the private key B<rsa> of type RSA

to B<ctx>. SSL_use_PrivateKey() adds B<pkey> as private key to B<ssl>;

SSL_use_RSAPrivateKey() adds B<rsa> as private key of type RSA to B<ssl>.

If a certificate has already been set and the private does not belong

to the certificate an error is returned. To change a certificate, private

key pair the new certificate needs to be set with SSL_use_certificate()

or SSL_CTX_use_certificate() before setting the private key with

SSL_CTX_use_PrivateKey() or SSL_use_PrivateKey(). 

SSL_CTX_use_PrivateKey_ASN1() adds the private key of type B<pk>

stored at memory location B<d> (length B<len>) to B<ctx>.

L<SSL_CTX_set_client_cert_cb(3)|SSL_CTX_set_client_cert_cb(3)>,

L<SSL_CTX_add_extra_chain_cert(3)|SSL_CTX_add_extra_chain_cert(3)>

=head1 HISTORY

Support for DER encoded private keys (SSL_FILETYPE_ASN1) in

SSL_CTX_use_PrivateKey_file() and SSL_use_PrivateKey_file() was added

in 0.9.8 .

=cut