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.h

@@ -4,6 +4,37 @@
 #include "specifier.h"
 #include "sysupdate-forward.h"
 
+typedef struct Context {
+        char *component;
+
+        Transfer **transfers;
+        size_t n_transfers;
+
+        Transfer **disabled_transfers;
+        size_t n_disabled_transfers;
+
+        Hashmap *features; /* Defined features, keyed by ID */
+
+        UpdateSet **update_sets;
+        size_t n_update_sets;
+
+        UpdateSet *newest_installed, *candidate;
+
+        Hashmap *web_cache; /* Cache for downloaded resources, keyed by URL */
+
+        int installdb_fd;
+} Context;
+
+Context* context_free(Context *c);
+DEFINE_TRIVIAL_CLEANUP_FUNC(Context*, context_free);
+
+typedef enum ReadDefinitionsFlags {
+        READ_DEFINITIONS_REQUIRES_ENABLED_TRANSFERS = 1 << 0,
+        READ_DEFINITIONS_REQUIRES_ANY_TRANSFERS     = 1 << 1,
+} ReadDefinitionsFlags;
+
+int context_make_offline(Context **ret, const char *node, const char *component, ReadDefinitionsFlags read_definitions_flags);
+
 extern bool arg_sync;
 extern uint64_t arg_instances_max;
 extern char *arg_root;