Add the getproxy command

On Windows systems there is a system proxy setting that is exposed by IE and Chrome. This commit adds a `getproxy` command to `stdapi` which pulls out the details of the proxy settings for the current user.

Sample test run without proxy settings:

```
meterpreter > getproxy
Auto-detect     : No
Auto config URL : 
Proxy URL       : 
Proxy Bypass    : 
```

Sample test run with all proxy settings configured:
```
meterpreter > getproxy
Auto-detect     : Yes
Auto config URL : http://baz:8080/foo.pac
Proxy URL       : foo:80
Proxy Bypass    : test;testagain;<local>
```

This command is not supported on POSIX.

This commit also adds another helper function called `packet_add_tlv_wstring` which was used by the `getproxy` innards. This is the same as `packet_add_tlv_string` but is for wide-character strings.

The Metasploit side of this PR is https://github.com/rapid7/metasploit-framework/pull/2592