Add documentation for the key logging callbacks

=pod

=head1 NAME

SSL_CTX_set_keylog_callback, SSL_CTX_get_keylog_callback,

SSL_CTX_keylog_cb_func - logging TLS key material

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 typedef void (*SSL_CTX_keylog_cb_func)(const SSL *ssl, const char *line);

 void SSL_CTX_set_keylog_callback(SSL_CTX *ctx, SSL_CTX_keylog_cb_func cb);

 SSL_CTX_keylog_cb_func SSL_CTX_get_keylog_callback(const SSL_CTX *ctx);

=head1 DESCRIPTION

SSL_CTX_set_keylog_callback() sets the TLS key logging callback. This callback

is called whenever TLS key material is generated or received, in order to allow

applications to store this keying material for debugging purposes.

SSL_CTX_get_keylog_callback() retrieves the previously set TLS key logging

callback. If no callback has been set, this will return NULL. When there is no

key logging callback, or if SSL_CTX_set_keylog_callback is called with NULL as

the value of cb, no logging of key material will be done.

The key logging callback is called with two items: the B<ssl> object associated

with the connection, and B<line>, a string containing the key material in the

format used by NSS for its B<SSLKEYLOGFILE> debugging output. To recreate that

file, the key logging callback should log B<line>, followed by a newline.

B<line> will always be a NULL-terminated string.

=head1 SEE ALSO

L<ssl(3)>

=head1 COPYRIGHT

Copyright 2016-2017 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