WL#13391: Enable the "system" mysql command line command for windows

The MySQL command line has a "system" command that executes its arguments as a
OS command and displays the result. This is a handy command. And it works well
on linuxes. But it's disabled on windows apparently because the CRT was not
supporting the necessary standard APIs and nobody cared to implement
alternatives for these.

Fast forward 10 years. Now the MSVC CRT supports all the necessary APIs, but the 
command is still disabled. And people do need it as evident from the frequent 
recurring updates in BUG#58680	Windows Clear Screen Command.

This is a worklog to re-enable this command for windows. 

FR1. system (\!) mysql command should be enabled just like on linux
FR2. the system (\!) mysql command should be visible in the command help 
FR3. the system command will execute on the same stdin,out,err as the mysql
command is executing.