MFC r312644, r312650

r312644:
Readd a feature lost in pw(8) refactoring

pw usermod foo -m

It used to be able to (re)create the home directory if it didn't exists

PR: 216224
Reported by: ae
MFC after: 3 days

r312650:
Really restore the old behaviour for pw usermod -m

It again reinstall missing skel files without overwriting changed one
Add a regression test about it

Reported by: ae
MFC after: 3 days