nuageinit: implement chpasswd

Add support for chpasswd, with all possible syntaxes, including
deprecated one: chpasswd.list as a list or as a multiline string
as some providers are still only providing this deprecated form

Sponsored by: OVHCloud
MFC After: 1 week
Reviewed by: kevans, jlduran
Differential Revision: https://reviews.freebsd.org/D50021