ShellPkg: CTRL-C stops a running script at the same time

This makes CTRL-C stop a running script after trying to stop the command.  And adds comments to describe the behavior more clearly.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Erik Bjorge <erik.c.bjorge@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15008 6f19259b-4bc3-4df7-8a09-765794883524