WL#7712: Support SSL by default in libmysql

Work is underway to enable SSL for all MySQL Server 5.7 deployments.  Similarly,
C connector should exhibit the following default behavior:

* Use SSL whenever it is supported by/enabled on the server
* Not validate server SSL certificates (hostname nor CA)

A hardened deployment will require (not just prefer) SSL, and will use
securely-deployed keys signed by a trusted CA.  All connectors should support
such a use case by offering modes exhibiting the following behavior:

* Require SSL.
* Validate hostname in server-supplied SSL certificate against the hostname to
which the connector is connecting.
* Validate the CA which signed the server certificate against the public key of
the CA, as supplied to the connector by the deployer.
* Support server-side x509 client certification verification by use of client
private key.

FR1 : If none of the SSL options are specified, libmysql client should attempt
an SSL connection if it is supported by server.

FR2 : If SSL connection is not possible as per FR1, libmysql client should fall
back to unencrypted connection.

IS 1 : Use/enforcement of SSL is controlled by use_ssl and enforce_ssl variables
in client library.
There are 3 possible combinations:

1. use_ssl=1, enforce_ssl=0 => Use SSL if available otherwise fallback to
unencrypted connection
2. use_ssl=1, enforce_ssl=1 => Enforce SSL connection and fail if one is not
established
3. use_ssl=0, enforce_ssl=0 => Use unencrypted connection

Current behavior is 3. After this worklog, default behavior would be option 1 above.

For clients other than mysqltest, this would mean:
Default (Without any ssl configuration option) : Use SSL if available otherwise
fallback to
unencrypted connection.
List of clients affected are:
mysql, mysqltest, mysql_config_editor, mysql_install_db, mysql_plugin,
mysql_secure_installation, mysql_upgrade, mysqladmin, mysqlbinlog, mysqlcheck,
mysqldump, mysqlimport, mysqlshow, mysqlslap.

IS 2: Backward compatibility

IS 2.1: Client which are explicitly configured
to skip SSL using either --skip-ssl or --ssl=0 shall not change from
previous behavior.
Client will always connect using unencrypted channel.

IS 2.2: Client which are explicitly configured
to SSL using either --ssl or --ssl=1 shall not change from previous behavior.
Client will always connect using ssl.

IS 2.3: In order to limit the impact of this change on tests, mysqltest should
attempt unecrypted connection by default. 

IS 2.4: If no ssl configuration is used (Without any ssl configuration option),
client will see a
change in behavior if server supports SSL connections as mentioned in IS 1.