Import latest mtree from NetBSD

Merge commit '7e59b238fcf32f3d365e78ddc702ca494e1ff68d'

This commit partially reverts the previous vendor import, given that the
"type" keyword has been historically mandatory and should not be removed
by "-R all". This was clarified in the man page.

Reported by: glebius
PR: 219467
MFC after: 1 week