install: consider non-Alias=/non-DefaultInstance= symlinks as "indirect" enablement

I think this matches the spirit of "indirect" well: the unit
*might* be active, even though it is not "installed" in the
sense of symlinks created based on the [Install] section.

The changes to test-install-root touch the same lines as in the previous
commit; the change in each case is from
   assert_se(unit_file_get_state(...) >= 0 && state == UNIT_FILE_ENABLED)
to
   assert_se(unit_file_get_state(...) >= 0 && state == UNIT_FILE_DISABLED)
to
   assert_se(unit_file_get_state(...) >= 0 && state == UNIT_FILE_INDIRECT)
in the last two commits.