Problem: In the mysqltest language, it was not possible to set the current
connection from a variable, and it was not possible to read the current
connection.
Fix: Allow setting the connection from a variable, like:
connection $variable;
and introduce the mysqltest language variable $CURRENT_CONNECTION, which
holds the name of the current connection.


client/mysqltest.cc:
  - Made select_connection use the common argument parser instead of its own
  home-rolled version. That allows variable expansion, for instance.
  - Made select_connection_name set the variable $CURRENT_CONNECTION, so that
  test scripts can use that.
  - Refactored a bit so that stuff that needs to be done when changing connection
  is located to one place.
mysql-test/t/mysqltest.test:
  Added test case for $CURRENT_CONNECTION and "connection $variable"