ShellPkg: Update CWD and current mapping when commands return

ShellPkg: Update CWD and current mapping when commands return

Some commands may cause current directory or even current mapping
become invalid. For example, "MAP" after unplugging the USB thumb
key may cause current mapping "FS0:" disappear.
This patch updates the CWD and current mapping when commands return.
It also causes the command prompt change to default "Shell>".

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Huajing Li <huajing.li@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by Jaben Carsey <jaben.carsey@intel.com>