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.

Sample run:

```
meterpreter > shell
[-] Failed to spawn shell with thread impersonation. Retrying without it.
Process 228 created.
Channel 4 created.
Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.

C:\Documents and Settings\Owner\Desktop>
```

Windows XP SP0 is the operating system version that was causing the problem for me.