importd: add new bus calls for importing local tar and raw images

This also adds "machinectl import-raw" and "machinectl import-tar" to
wrap these new bus calls.

THe commands basically do for local files that "machinectl pull-raw" and
friends do for remote files.

src/shared/machine-pool.h

@@ -23,5 +23,8 @@
 
 #include "sd-bus.h"
 
+/* Grow the /var/lib/machines directory after each 10MiB written */
+#define GROW_INTERVAL_BYTES (UINT64_C(10) * UINT64_C(1024) * UINT64_C(1024))
+
 int setup_machine_directory(uint64_t size, sd_bus_error *error);
 int grow_machine_directory(void);