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: rename prompt() to ask_new_field()

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
chfn: remove set_changed_data() and add add_missing()

The add_missing() and find_field() functions are needed when input data
is incomplete, such as in case when chfn is instructed to change only
selected fields with command line options.

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>