Reject mount destination path traversal

[AliAltivate]

Summary

Reject mount destinations containing .. path components in both the legacy and new mount APIs.

Security impact

MountPt.dst is documented as the mount point inside the jail, but the legacy path-based implementation and the new openat-based implementation accepted traversal components. A crafted mount configuration such as tmp/../../outside_marker could resolve outside the jail root. With a non-mandatory src_content mount, this allowed a file to be written outside the root while nsjail continued successfully.

The issue was reproduced locally in a privileged container built from this repository. The same probe affected both mount implementations.

Fix

• Add a shared mount-destination validator.
• Reject .. components before path or mount side effects in both APIs.
• Add focused regression checks for both legacy and new mount modes.

Validation

• Clean build with -Wall -Wextra -Werror.
• Pre-fix escape reproduced in both APIs.
• Post-fix escape blocked in both APIs.
• Existing true/false sanity checks continue to pass.
• git diff --check passes.

This change intentionally preserves existing handling of other destination components and is limited to preventing lexical path traversal.

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