os module: Get current user/group

[dnbsd]

In my scripts I would like to be able to obtain current user's name and group. Go's stdlib has a package os/user which provides the necessary functionality and Risor module could be based on. I am proposing to add the following new functions to the os module:

• current_user() user
• lookup_user(name) user
• lookup_uid(uid) user
• lookup_group(name) group
• lookup_gid(gid) group

With this change, I would also like to extend the OS interface so that their implementation can be changed by a custom OS implementation, similarly how it's already done for other functions in Risor's os module:

type User interface {
    func Uid() string
    func Gid() string
    func Username() string
    func Name() string
    func HomeDir() string
}

type Group interface {
    func Gid() string
    func Name() string
}

type OS interface {
    // ...
    func CurrentUser() User
    func LookupUser(name string) User
    func LookupUid(uid string) User
    func LookupGroup(name string) Group
    func LookupGid(gid string) Group
}

If this is agreeable, @myzie, I would like to take a shot at it.