Retry shell without thread impersonation

In certain scenarios on Windows XP there are times when creating a
shell fails with the error `ERROR_PRIVILEGE_NOT_HELD`. When this
happens the user will usuall fallback to a non-impersonated shell
via the command: `execute -f cmd.exe -H -i -c`

This patch catches the error, warns the use of the failure and then retries
to create the interactive shell without the `-t` flag.
Land #2555 - Retry shell without thread impersonation