mount2: add --direct-mount flag to prevent triggering fusermount SELi… - #10025

What does this change do?

Provides the --direct-mount flag with rclone mount2, and the option is not supported by rclone mount because legacy mount library does not support anything like this :

rclone mount2 --direct-mount remote:path /path/to/mountpoint

This flag makes rclone mount the filesystem directly via syscall.Mount instead of forking fusermount3. There is no fd transfer, no socketpair, and no behaviour SELinux would treat as a violation

By the way, --direct-mount requires access to /dev/fuse and of course appropriate permissions (root, or CAP_SYS_ADMIN plus proper AppArmor or seccomp configuration. I run it as root so these are validated for me.

I have used a coding agent to help me with this issue/PR.

I have run make quicktest, and all passed (once i put rclone in PATH or TestEndToEnd would fail)

I have tested this against the system which triggered the issue (debian 13 with SELinux enabled) and it solved my problem : rclone starts and works immediately even with SELinux enabled with an unchanged policy.

This is my first contribution to rclone, and i am new to go. Do not hesitate to tell me if anything needs changing.

Linked issue

#10024

Checklist

• This change is trivial OR it has been discussed and agreed in the linked issue.
• I have read the contribution guidelines.
• (If I used AI tools to help write this code) I have read and understood the AI-assisted contributions guidance, and I have tested and take ownership of this change myself.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate.
• All commit messages are in house style.
• (Backend changes only) test_all passes for this backend and if submitting a new backend can provide a test account for the integration tester - see CONTRIBUTING.md.
• This Pull Request is ready for review.