Align the filesystem to 512 bytes and allow paddings to allow mounting via loop devices.

When trying to set up a `dwarfs` file as a loop device, it does not align to the default 512B sector boundary. However, if I pad the file to multiples of 512B, it also fail to mount.
This affects automounting utilities like `udevil`, which may be used in GUI file managers and such. Another use case is when the `dwarfs` is written to a dedicated partition (in a firmware image, I imagine), which is likely to have paddings after the filesystem itself.
```
0 ~:ls -l test.dwarfs
-rw-r--r-- 1 root root 45834020970 03-28 11:28 test.dwarfs
0 ~:mkdir test
0 ~:mount -t fuse.dwarfs test.dwarfs test
0 ~:umount test
0 ~:udevil mount --verbose -t fuse.dwarfs test.dwarfs
ROOT: /sbin/losetup /dev/loop0 /root/test.dwarfs
losetup: /root/test.dwarfs: Warning: file does not end on a 512-byte sector boundary; the remaining end of the file will be ignored.
ROOT: /bin/mount -v -t fuse.dwarfs -o nosuid,noexec,nodev,lazytime,ro /dev/loop0 /run/mount/test.dwarfs
E 11:26:54.892551 error initializing file system: dwarfs::runtime_error: [fs_section.cpp:68] truncated section data: 45834020970 > 45834020864
mount exit status = 1
ROOT: /sbin/losetup -d /dev/loop0
0 ~:echo $((45834020970%512))
106
0 ~:echo $((45834020970-106+512))
45834021376
0 ~:truncate -s 45834021376 test.dwarfs
0 ~:udevil mount --verbose -t fuse.dwarfs test.dwarfs
ROOT: /sbin/losetup /dev/loop0 /root/test.dwarfs
ROOT: /bin/mount -v -t fuse.dwarfs -o nosuid,noexec,nodev,lazytime,ro /dev/loop0 /run/mount/test.dwarfs
E 11:29:02.360347 error initializing file system: dwarfs::runtime_error: [fs_section.cpp:52] truncated section header: 45834021354 + 64 > 45834021376
mount exit status = 1
ROOT: /sbin/losetup -d /dev/loop0
```
Also, please consider adding a filesystem label and UUID, so the mountpoint can be displayed better in GUI file managers.