91fe922f05
when installing hard links. They have no effect except when using a
metalog, in which case the information is added to the metalog. In
the future, these variables may be replaced by a method for explicitly
recording hard links in a metadata log.
Also change a few things that called ${INSTALL_LINK} without going
through bsd.links.mk.
Reviewed by perry and joerg. This should fix PR 24457 and PR 41155.