Implement SSL_CTX_set_msg_callback/SSL_set_msg_callback for ssl/tls record tracing

[CowboyTim]

This adds support for the msg callback functionality to do some debugging for e.g. tls handshake.

• lib/Net/SSLeay.pod: update manpage
• updated Changes
• updated MANIFEST
• t/local/46_msg_callback.t added

[h-vn]

To help with observing the messages in the callback function, future work could include:

• adding constants listed on the manual page https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_msg_callback.html
• providing helper functions within SSLeay.xs to convert record content type (Handshake, Alert, etc.) to strings
• similar helpers for converting Handshake message types (ClientHello, Finished, etc.) to strings
• likewise for Alerts for stringifying Level and Description

Here's an example from Curl:

* TLSv1.2 (OUT), TLS handshake, Client hello (1):
* TLSv1.2 (IN), TLS handshake, Server hello (2):
* TLSv1.2 (IN), TLS handshake, Certificate (11):
* TLSv1.2 (IN), TLS handshake, Server key exchange (12):
* TLSv1.2 (IN), TLS handshake, Server finished (14):
* TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
* TLSv1.2 (OUT), TLS change cipher, Change cipher spec (1):
* TLSv1.2 (OUT), TLS handshake, Finished (20):
* TLSv1.2 (IN), TLS change cipher, Change cipher spec (1):
* TLSv1.2 (IN), TLS handshake, Finished (20):

[CowboyTim]

Thx!
Yes, those constants etc,.. are indeed next on my todo list - but it will depend on time and availability. They are of less importance but it would be nice to have these constants and functions implemented as it makes this feature more complete and matching openssl/curl functionality.