Add a replacement for sqfs_writer_t in libsquashfs

[AgentD]

The libcommon.a helper library currently contains an sqfs_writer_t implementation that uses libsquashfs and libfstree together to provide a simple, high level interface for creating squashfs images.

It would benefit users of libsquashfs to build a somewhat more generic replacement into libsquashfs.

There are still a few points that need to be ironed out for the interface design:

• Given that the interface would be much more high-level/opaque, how to properly generate error messages.
• At some point, it might be extended to not just generating squashfs images, but to read an existing image and allow editing it.
• Would it be reasonable to also add a dead-simple filesystem like API to read from squashfs images? Should that include caching? Asynchronous read & decompress?
• Should the two somehow be able to reasonably interoperate? If so, how?

Of course, some of the later points are not yet relevant as long as simple editing, or a high-level reader is not planned, but should be kept in mind to make API extensible enough.

This move has so far not been done for two reasons:

• Open questions regarding the design of a proper API/ABI for it.
• The lack of a need, as it was assumed that most projects doing filesystem manipulation would have their own high-level layer that libsquashfs would be integrated into.

[AgentD]

An appropriate API for the writer would probably be shaped something like this:

• Create from a description struct
• writer_mkdir/writer_mknod/writer_symlink/...
• writer_begin_file, writer_append_file_data, writer_end_file
• writer_begin_xattr, writer_append_xattr, writer_end_xattr
• flush to disk