command line clients.

Command line tools like mysqladmin and mysqldump did not recognize
default-auth and plugin-dir options.

Support for these options was found missing in these command line
tools.

Fixed by adding support for the same.


client/mysqladmin.cc:
  Bug#58139 : default-auth option not recognized in MySQL standard
              command line clients.
  
  Introduced two new variables to hold values from default-auth and
  plugin-dir options and further pushed them to client's st_mysql
  instance.
client/mysqldump.c:
  Bug#58139 : default-auth option not recognized in MySQL standard
              command line clients.
  
  Introduced two new variables to hold values from default-auth and
  plugin-dir options and further pushed them to client's st_mysql
  instance.
mysql-test/r/plugin_auth.result:
  Added test case for Bug#58139.
mysql-test/t/plugin_auth.test:
  Added test case for Bug#58139.