Add check_key_exists registry function

MSF side has been attempting to open keys to see if they exist, which isn't fantastic as it results in an error. This change adds a function which indicates to the caller if the given reg key exists.

I also updated the `stdapi_net_config_get_proxy` command declaration to use the appropriate macro.

### Verification

- [x] Check missing key works on x86
- [x] Check existing key works on x86
- [x] Check missing key works on x64
- [x] Check existing key works on x64

MSF side of this PR is [over here](https://github.com/rapid7/metasploit-framework/pull/2686)