Add some documentation for the OCSP callback functions

=pod

=head1 NAME

SSL_CTX_set_tlsext_status_cb, SSL_CTX_set_tlsext_status_arg,

SSL_set_tlsext_status_type, SSL_get_tlsext_status_ocsp_resp,

SSL_set_tlsext_status_ocsp_resp - OCSP Certificate Status Request functions

=head1 SYNOPSIS

 #include <openssl/tls1.h>

 long SSL_CTX_set_tlsext_status_cb(SSL_CTX *ctx,

                                   int (*callback)(SSL *, void *));

 long SSL_CTX_set_tlsext_status_arg(SSL_CTX *ctx, void *arg);

 long SSL_set_tlsext_status_type(SSL *s, int type);

 long SSL_get_tlsext_status_ocsp_resp(ssl, unsigned char **resp);

 long SSL_set_tlsext_status_ocsp_resp(ssl, unsigned char *resp, int len);

=head1 DESCRIPTION

A client application may request that a server send back an OCSP status response

(also known as OCSP stapling). To do so the client should call the

SSL_set_tlsext_status_type() function prior to the start of the handshake.

Currently the only supported type is B<TLSEXT_STATUSTYPE_ocsp>. This value

should be passed in the B<type> argument. The client should additionally provide

a callback function to decide what to do with the returned OCSP response by

calling SSL_CTX_set_tlsext_status_cb(). The callback function should determine

whether the returned OCSP response is acceptable or not. The callback will be

passed as an argument the value previously set via a call to

SSL_CTX_set_tlsext_status_arg(). Note that the callback will not be called in

the event of a handshake where session resumption occurs (because there are no

Certificates exchanged in such a handshake).

The response returned by the server can be obtained via a call to

SSL_get_tlsext_status_ocsp_resp(). The value B<*resp> will be updated to point

to the OCSP response data and the return value will be the length of that data.

Typically a callback would obtain an OCSP_RESPONSE object from this data via a

call to the d2i_OCSP_RESPONSE() function. If the server has not provided any

response data then B<*resp> will be NULL and the return value from

SSL_get_tlsext_status_ocsp_resp() will be -1.

A server application must also call the SSL_CTX_set_tlsext_status_cb() function

if it wants to be able to provide clients with OCSP Certificate Status

responses. Typically the server callback would obtain the server certificate

that is being sent back to the client via a call to SSL_get_certificate();

obtain the OCSP response to be sent back; and then set that response data by

calling SSL_set_tlsext_status_ocsp_resp(). A pointer to the response data should

be provided in the B<resp> argument, and the length of that data should be in

the B<len> argument.

=head1 RETURN VALUES

The callback when used on the client side should return a negative value on

error; 0 if the response is not acceptable (in which case the handshake will

fail) or a positive value if it is acceptable.

The callback when used on the server side should return with either

SSL_TLSEXT_ERR_OK (meaning that the OCSP response that has been set should be

returned), SSL_TLSEXT_ERR_NOACK (meaning that an OCSP response should not be

returned) or SSL_TLSEXT_ERR_ALERT_FATAL (meaning that a fatal error has

occurred).

SSL_CTX_set_tlsext_status_cb(), SSL_CTX_set_tlsext_status_arg(),

SSL_set_tlsext_status_type() and SSL_set_tlsext_status_ocsp_resp() return 0 on

error or 1 on success.

SSL_get_tlsext_status_ocsp_resp() returns the length of the OCSP response data

or -1 if there is no OCSP response data.

=cut