Add libfuse 3 support with libfuse 2 fallback

[matejk]

Summary

Port of LTFS to libfuse 3 with a compile-time libfuse 2 fallback, plus the FUSE 3 performance features. Based on the v2.4.8.4 release on main, 3 commits (the port, the performance features, macOS support). The test suite, the bug fixes found during this work, and the CMake build are separate independent PRs.

FUSE 3 support

• Builds against fuse3 (>= 3.4) by default on Linux, with automatic fallback to fuse2 when fuse3 is absent; --with-fuse2 forces the legacy API (still the default on macOS and the BSDs).
• Handlers use FUSE 3 signatures: getattr/truncate absorb the handle variants, chmod/utimens use the file handle when the path is NULL (nullpath_ok), rename handles RENAME_NOREPLACE/RENAME_EXCHANGE, use_ino/hard_remove/nullpath_ok move into fuse_config, and FUSE_CAP_ASYNC_READ is cleared to keep tape reads ordered (-o sync_read no longer exists).
• fuse_file_info.parallel_direct_writes is feature-detected (the field appeared in libfuse 3.15; version checks broke on ubuntu-24.04's 3.14).
• macOS: with macFUSE 5 (which ships a libfuse 3), the FUSE 3 build works via FUSE_DARWIN_ENABLE_EXTENSIONS=0; macOS keeps fuse2 as its default.

Performance

• 1 MiB FUSE requests (-o max_write=N): measured 1 MiB writes and O_DIRECT reads reaching the daemon, vs 128 KiB on fuse2.
• -o direct_io: all I/O bypasses the kernel page cache; ~70 % higher sequential write throughput on the file backend and a flat page cache during large streaming jobs.
• readdirplus: ls -l over 100 files needs 2 getattr requests instead of 102 (effective with libfuse >= 3.17).

Relationship to the other PRs

• Integration tests for these features (request sizes, direct_io, readdirplus) are part of the test-suite PR (Add an integration test suite using the file tape backend #611).
• Merge-order note: the scheduler data-loss fix Fix data loss on writes larger than the tape block size #607 (approved) should merge before or together with this PR — FUSE 3's 1 MiB requests expose that bug in the default configuration.

Verification

With #607 and #611 merged alongside: all 14 integration tests pass with fuse3 and fuse2, via make check and ctest, against libfuse 3.14 and 3.17; zero compiler warnings at -Wall -Wextra; macOS compile-verified for both APIs against the macFUSE 5.2 SDK.

Issues

• Addresses the FUSE API half of Build Failure: FUSE API error and deprecated pthread_yield in CentOS-Stream-9-latest-x86_64-dvd1.iso #465 (the pthread_yield half is in Build and CLI fixes #608).
• Prerequisite for io_uring support (io_uring support #468).
• The fuse3-with-fallback configure also covers environments without a fuse2 package (Devuan Chimaera Build Errors and Warnings #393).

[vandelvan]

Hey @matejk ! Thank you for contributing, there are a couple of things in this PR that seem to be part of your other PRs, please remove/move the extra changes as they are not related to fuse3

[matejk]

Done. The data-loss change is removed (it stays in #607) and the test scripts are removed (they stay in #611); this PR is now fuse3-only, 3 commits. One note: #607 should merge before or together with this PR, because FUSE 3's 1 MiB requests trigger that bug.

[vandelvan]

Thank you for your contribution! LGTM