Add the getproxy command

[OJ]

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 rapid7/metasploit-framework#2592