vipw: use explicit_bzero() to clear buffer after copy

vipw: use explicit_bzero() to clear buffer after copy

This should make leaking end of /etc/shadow file more unlikely.

Notice that there is now way to tell to editors they should ensure none it
does not leak any buffers, drop cores, and so on, when editing sensitive
data.  In short this change is addressing the issue only partially.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>