ShellPkg/UefiShellLib.c: Execute: Return a Command status even in the old shell

This means we can use ShellExecute without thinking which shell
environment is in use. However it still isn't ideal: if
mEfiShellEnvironment2->Execute returns EFI_INVALID_PARAMETER, we can't tell
whether Execute() received an invalid parameter (e.g. ParentImageHandle was
NULL), or whether we tried to execute a command with an invalid parameter
(for example CommandLine "ls -hurdygurdy").

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Brendan Jackman <Brendan.Jackman@arm.com>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
Reviewed-by: Jaben Carsey <Jaben.carsey@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15183 6f19259b-4bc3-4df7-8a09-765794883524