sysupdate: introduce "installdb" that keeps track of installed resources

Let's make sure we keep track of any file we drop into the system via a
database in /var/. This database is implemented based on symlinks, i.e.
reuses the fs as a simple database. Given the database most likely will
have <= 10 entries only (as we store *patterns* of installed file paths in
them, not the file paths themselves), this should be very efficient.

For implementation details see comments at top of
src/sysupdate/sysupdate-cleanup.c.

src/sysupdate/sysupdate-transfer.c

@@ -35,6 +35,7 @@
 #include "strv.h"
 #include "sync-util.h"
 #include "sysupdate.h"
+#include "sysupdate-cleanup.h"
 #include "sysupdate-feature.h"
 #include "sysupdate-instance.h"
 #include "sysupdate-pattern.h"
@@ -1672,6 +1673,8 @@ int transfer_install_instance(
                          resource_type_to_string(t->target.type));
 
                 t->temporary_pending_path = mfree(t->temporary_pending_path);
+
+                (void) context_installdb_record(t->context, t->target.path, t->target.patterns);
         }
 
         if (t->final_partition_label) {