mount: clean up mount APIs

Change the Repository::mount() API to return the mounted filesystem as
an fd rather than taking the mountpoint as an argument.  Create a new
mount_at() API to replace the old one, replacing the canicalize() and
mount_at() calls that used to be in mount_composefs_at(), which we
remove.

Update the various users.

Making this change lets us simplify the logic in composefs-setup-root:
it no longer has to manually open the image in order to perform the
fsmount operation: it can use the new API on the repository.

This allows us to make Repository::open_image() private, so do that too.

Co-authored-by: Sanne Raymaekers <[email protected]>
Signed-off-by: Allison Karlitskaya <[email protected]>

Author: allisonkarlitskaya

src/main.rs

@@ -339,7 +339,7 @@ async fn main() -> Result<()> {
             fs.print_dumpfile()?;
         }
         Command::Mount { name, mountpoint } => {
-            repo.mount(&name, &mountpoint)?;
+            repo.mount_at(&name, &mountpoint)?;
         }
         Command::ImageObjects { name } => {
             let objects = repo.objects_for_image(&name)?;