[Toybox] Sending passwd source code for contribution

[orc]

> (One thing it probably should do is
> check if it's running suid root and exit if not, rather than
> prompting for a password and then failing to be able to change it...)

On tcb-enabled system this maybe an undesired behavior. I run such
system as a test environment, busybox passwd of course exits without
root privs, but user can change his password by
editing /etc/tcb/$USER/shadow if allowed (built-in musl support in such
routines as getspnam()).

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