expand path of systemctl link argument (#6186)

expand path of systemctl link argument (#6186)

systemctl link is the only systemctl verb that takes a filename (and not
a unit name) as argument

use path_strv_make_absolute_cwd to expand the provided filename in order
to make it easier to use from the command line

keep the absolute pathname requirement when --root is used

[zj: add explicit error messages for the cases of --root and plain filename
instead of skipping normalization and just relying on systemd to refuse
to link non-absolute arguments. This allows us to make the error message
more informative.]