avoid sending pointer values in struct passwd

[Peter Stuge]

Yuichiro NAITO wrote:
> Take a look at my GitHub pull request to see my patch.
> 
> https://github.com/openssh/openssh-portable/pull/216

I think the length at the beginning should be tied to the (number of?)
members that are sent instead of the struct passwd size on either side.

Also, adding passwd.fields seems to be an unrelated change, better
placed in a separate commit.

Finally, sshbuf_free_passwd() is added but never called. If it is not
needed then I think it's better to not add it (yet).


//Peter