Support -host/-currentHost options to defaults

`defaults` allows either `-currentHost` or `-host [hostname]` to be given as the first argument. This is required to set certain system-wide values on OSX, e.g. NSGlobalDomain com.apple.mouse.tapBehavior:

```
$ defaults -currentHost read NSGlobalDomain com.apple.mouse.tapBehavior
0
$ defaults read NSGlobalDomain com.apple.mouse.tapBehavior                                                                                                                                                        2013-03-09 10:43:26.882 defaults[72946:707]
The domain/default pair of (kCFPreferencesAnyApplication, com.apple.mouse.tapBehavior) does not exist
```