pw: set the user's home directory mode if it existed

The adduser(8) prompt allows one to set the mode of a new home
directory, but pw(8) doesn't honor the -M mode if the home directory
already exists at creation time. It doesn't seem to make sense to
ignore the mode (which may lead to a security issue on the system being
configured) when we'll happily chown an existing directory, so fix the
inconsistency.

PR: 280099
Reviewed by: des, jlduran (previous version)
Differential Revision: https://reviews.freebsd.org/D46443