Fix warnings related to the use of the deprecated gets() function
and passing NULL to non-pointer argument of the sys_var constructor.
plugin/auth/dialog.c:
Do not use the deprecated gets() function.
sql/sys_vars.h:
Do not pass NULL to a non-pointer argument of the sys_var constructor.
connectors plugins
Implemented changes needed to keep the client plugin API compatible with
the existing plugins :
1. Provided an options() client plugin API to let the application pass
options to the plugin after loading it
2. Added "License" (const char *) to specify the client plugin's license
3. Added "mysql_api" as a placeholder that the client library can use
to pass function pointers to the plugin so that the plugin can call the
C lib back.
4. Updated the existing client plugins to comply with the API change.
5. Added more detailed error message generation for Windows.
