ShellPkg: Per UEFI Shell 2.2 SPEC to make Shell supports 'NoNesting'.

ShellPkg: Per UEFI Shell 2.2 SPEC to make Shell supports 'NoNesting'.

This patch makes Shell support -nonesting invocation option. This option
specifies that EFI_SHELL_PROTOCOL.Execute API nesting of a new Shell
instance is optional and dependent on the 'nonesting' Shell environment
variable.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Jim Dailey <Jim_Dailey@Dell.com>