Add documentation for the ability to control the number of tickets

=pod

=head1 NAME

SSL_set_num_tickets,

SSL_get_num_tickets,

SSL_CTX_set_num_tickets,

SSL_CTX_get_num_tickets

- control the number of TLSv1.3 session tickets that are issued

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 int SSL_set_num_tickets(SSL *s, size_t num_tickets);

 size_t SSL_get_num_tickets(SSL *s);

 int SSL_CTX_set_num_tickets(SSL_CTX *ctx, size_t num_tickets);

 size_t SSL_CTX_get_num_tickets(SSL_CTX *ctx);

=head1 DESCRIPTION

SSL_CTX_set_num_tickets() and SSL_set_num_tickets() can be called for a server

application and set the number of session tickets that will be sent to the

client after a full handshake. Set the desired value (which could be 0) in the

B<num_tickets> argument. Typically these functions should be called before the

start of the handshake.

The default number of tickets is 2; the default number of tickets sent following

a resumption handshake is 1 but this cannot be changed using these functions.

The number of tickets following a resumption handshake can be reduced to 0 using

custom session ticket callbacks (see L<SSL_CTX_set_session_ticket_cb(3)>).

Tickets are also issued on receipt of a post-handshake certificate from the

client following a request by the server using

L<SSL_verify_client_post_handshake(3)>. These new tickets will be associated

with the updated client identity (i.e. including their certificate and

verification status). The number of tickets issued will normally be the same as

was used for the initial handshake. If the initial handshake was a full

handshake then SSL_set_num_tickets() can be called again prior to calling

SSL_verify_client_post_handshake() to update the number of tickets that will be

sent.

SSL_CTX_get_num_tickets() and SSL_get_num_tickets() return the number of

tickets set by a previous call to SSL_CTX_set_num_tickets() or

SSL_set_num_tickets(), or 2 if no such call has been made.

=head1 RETURN VALUES

SSL_CTX_set_num_tickets() and SSL_set_num_tickets() return 1 on success or 0 on

failure.

SSL_CTX_get_num_tickets() and SSL_get_num_tickets() return the number of tickets

that have been previously set.

=head1 HISTORY

These functions were added in OpenSSL 1.1.1.

=head1 COPYRIGHT

Copyright 2018 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License").  You may not use

this file except in compliance with the License.  You can obtain a copy

in the file LICENSE in the source distribution or at

L<https://www.openssl.org/source/license.html>.

=cut