chfn: move new and old finger structs to chfn control struct

This change is a little bit messy, and requires a comment the struct
finfo should not have 'struct passwd *pw' as it's member.  The earlier
struct design would have been burden to maintain, and confusing to use.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
chfn: make command to obey login.defs CHFN_RESTRICT instructions

Reference: http://man7.org/linux/man-pages/man5/login.defs.5.html
Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=138519
Signed-off-by: Sami Kerola <kerolasa@iki.fi>