Fix missing files in METALOG with -DNO_ROOT

By using INSTALL_LINK instead of calling ln during install the files
end up in the METALOG file as well if we use -DNO_ROOT and will be
included in a disk image when using makefs with METALOG as the input.
The other file that was not included in METALOG was /var/db/services.db
which is now also included for -DNO_ROOT.

Approved By: brooks (mentor)
Differential Revision: https://reviews.freebsd.org/D15665