fs: prefer fusermount3; add nonempty when using fusermount (FUSE2)

[delandtj]

Summary:

• Prefer fusermount3 when available; fallback to fusermount.
• Pass nonempty only when using legacy fusermount (FUSE2) to allow mounting over non-empty directories.

Key changes:

• Mount options baseline (without nonempty): src/fs/mod.rs:76
• Prefer fusermount3, fallback to fusermount, using absolute path per polyfuse: src/fs/mod.rs:82
• Clone path before inspection: src/fs/mod.rs:86
• Conditionally add nonempty only for fusermount (FUSE2): src/fs/mod.rs:91

Build:

• Verified with MUSL target debug build using feature flag:
  cargo build --target x86_64-unknown-linux-musl --features build-binary

Context:

• Addresses mounting over non-empty directories while maintaining FUSE3 compatibility (nonempty omitted for fusermount3).