apps: Add support for writing a keylog file

The server and client demos (s_client and s_server) are extended with a
-keylogfile option. This is similar as setting the SSLKEYLOGFILE
environment variable for NSS and creates a keylog file which is suitable
for Wireshark.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2343)