lock-util: make global lock return parameter to image_path_lock() optional

When adding unprivileged nspawn support we don't really want a global
lock file, since we cannot even access the dir they are stored in, hence
make the concept optional.

Some minor other modernizations.

src/basic/lock-util.h

@@ -17,7 +17,7 @@ static inline int make_lock_file(const char *p, int operation, LockFile *ret) {
 int make_lock_file_for(const char *p, int operation, LockFile *ret);
 void release_lock_file(LockFile *f);
 
-#define LOCK_FILE_INIT { .dir_fd = -EBADF, .fd = -EBADF }
+#define LOCK_FILE_INIT (LockFile) { .dir_fd = -EBADF, .fd = -EBADF }
 
 /* POSIX locks with the same interface as flock(). */
 int posix_lock(int fd, int operation);