Fix /proc dispatch for host-passthrough dirfds

[jserv]

Guest find/ls on /proc failed because host-kernel FDs from CONTINUE'd openat on virtual paths were not recognized in subsequent *at syscalls. The path translator resolved relative paths against CWD, making them absolute, causing resolve_open_dirfd to ignore the dirfd entirely.

Detect host-passthrough dirfds in translate_request_at_path: when dirfd is not AT_FDCWD, non-negative, and not tracked in the FD table, read the guest path and signal CONTINUE for safe relative lookups. Reject paths containing ".." components or /proc magic symlinks (root, cwd, exe, fd, fdinfo, map_files) that could escape the virtual namespace.

Add should_continue_virtual_path to gate CONTINUE on originally-absolute paths, preventing relative paths like "./proc" from bypassing LKL.

Move kbox_relative_path_has_dotdot to path.c as public API with unit tests covering dotdot detection and proc escape patterns including fd, fdinfo, and map_files.

Change-Id: Icb46feda38b8370a610c31b55ada9368bea357af

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Summary by cubic

Fixes /proc dispatch by detecting host-passthrough dirfds and gating virtual-path continuations, restoring tools like find and ls under /proc. Blocks unsafe relative lookups to prevent escaping the virtual namespace.

• Bug Fixes

  • Detect untracked host dirfds in translate_request_at_path; for relative paths set CONTINUE, but reject .. and /proc escape names (root, cwd, exe, fd, fdinfo, map_files); detect /proc dirfds via /proc/<pid>/fd/<n>.
  • Gate virtual-path CONTINUE to originally-absolute guest paths with should_continue_virtual_path, preventing ./proc from bypassing LKL; applied to open*, stat*, faccessat, newfstatat, and legacy path calls.
  • Apply host-dirfd handling to readlinkat, symlinkat, linkat, and utimensat so safe relative lookups continue to the host.

• Refactors

  • Expose kbox_relative_path_has_dotdot and add kbox_relative_proc_escape_path in path.c.
  • Extend kbox_is_proc_escape_path to flag fd, fdinfo, and map_files; add unit tests for .. detection and /proc escape patterns (including relative forms).

^{Written for commit 0000a3b. Summary will update on new commits.}