Support for file filtering (hiding)

[Nekun]

This new feature allows to hide files specified by a slash-separated list, --file-filter=r:*.png/d:dir1/junkfile/..., optionally POSIX globs and matching file modes can be specified. Files not appears in all directories, any access-by-path ops fails with ENOENT, any "write" ops, such as rename and link, fails with EPERM, even if specified files not exists.

Developed this feature for QEMU shared filesystems: bindfs is very useful for mapping owners and permissions when QEMU runs unprivileged and can't chown, but with "mapped-file" option (https://wiki.qemu.org/Documentation/9psetup#Starting_the_Guest_directly) which is only option for tmpfs since it can't store xattrs metadata is stored in target path in ".virtfs_metadata" dir created in each directory, makes it very inconvenient to delete them while copying files from guest to host. With filtering feature, we can just filter metadata dirs in resulting share.

[mpartel]

I'm sorry I've still not found the time for this. It's still on my list.