CONNECTIONS IF SPE

Problem description: -ssl-key value is not validated, you can assign any bogus 
text to --ssl-key and it is not verified that it exists, and more importantly, 
it allows the client to connect to mysqld.

Fix: Added proper validations checks for --ssl-key.

Note:
1) Documentation changes require for 5.1, 5.5, 5.6 and trunk in the sections
   listed below and the details are :

 http://dev.mysql.com/doc/refman/5.6/en/ssl-options.html#option_general_ssl
    and
 REQUIRE SSL section of
 http://dev.mysql.com/doc/refman/5.6/en/grant.html

2) Client having with option '--ssl', should able to get ssl connection. This 
will be implemented as part of separate fix in 5.6 and trunk.