shared/install: handle dangling aliases as an explicit case, report nicely

This fixes 'preset-all' with a unit that is a dangling symlink.

$ systemctl --root=/ preset-all
Unit syslog.service is an alias to a unit that is not present, ignoring.
Unit auditd.service is masked, ignoring.
Unit NetworkManager.service is masked, ignoring.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/shared/install.h b/src/shared/install.h
index 5812447c5b..c6aa4f6ef1 100644
--- a/src/shared/install.h
+++ b/src/shared/install.h
@@ -73,6 +73,7 @@ enum UnitFileChangeType {
         UNIT_FILE_SYMLINK,
         UNIT_FILE_UNLINK,
         UNIT_FILE_IS_MASKED,
+        UNIT_FILE_IS_DANGLING,
         _UNIT_FILE_CHANGE_TYPE_MAX,
         _UNIT_FILE_CHANGE_INVALID = INT_MIN
 };