WIP: Add mount command

[aawsome]

This PR adds the mount and the webdav command to rustic to access snapshot contents as a read-only filesystem.

For BOTH commands there are following options:

• Access to a concrete snapshot/path, e.g. rustic mount /mnt 37a63e5b:/my/path.
• Access to all snapshots (maybe restricted by filters) by using templates to define a virtual directory structure where snapshots are located. Example: rustic webdav 127.0.0.1:4999 --path-template "[{hostname}]/[{label}]/{time}" --time-template "%Y-%m-%d_%H-%M-%S" (these are also defined as default). Note that for all dirs containing only snapshots, also a latest entry is generated. latest and identical subsequent snapshots are symlinks when using mount and optionally symlinks when using webdav.

Depends on rustic-rs/rustic_core#106 and is based on #966

This PR uses fuse_mt which is not optimal as it introduces some overhead (e.g. needs to save whole Paths in-memory).

TODO:

• use blob cache
• handle hot/cold repositories (allow only directory listing unless explicitely specified)

[jjedelsky]

Would it be possible to add additional options from command line?
Eq. rustic mount -o allow_other or rustic mount -o default_permissions.

[aawsome]

Sure. Also if there are sensible defaults, we should try to use them. What are good defaults in you opinion?

[jjedelsky]

Imho, kernel_cache, allow_other, and default_permissions are going to be the right choice most of the time.

(For example, without allow_other, a fs mounted in toolbox (or any other container) would appear empty in gui file manager running outside of the container)

It probably might be sometimes useful to disable allow_other and default_permissions.

[aawsome]

I'll add kernel_cache to the options.

I am wondering if we should just add an option like _no_allow_other to the mount command which then unsets both allow_other and default_permission - and without this option just set both...

[jjedelsky]

That sounds good.