path-util: add a function to peek into a container and guess systemd version

This is a bit crude and only works for new systemd versions which
have libsystemd-shared.

src/basic/path-util.h

@@ -125,3 +125,5 @@ char *file_in_same_dir(const char *path, const char *filename);
 bool hidden_or_backup_file(const char *filename) _pure_;
 
 bool is_device_path(const char *path);
+
+int systemd_installation_has_version(const char *root, unsigned minimal_version);