Use per-device dynamic devfs numbers

[lacraig2]

Summary

• remove the shared dynamic portal major/minor allocator for synthetic devfs devices
• restore per-device kernel dynamic allocation for automatic char and block devices
• keep explicit Python-provided major/minor registrations authoritative

Testing

• ./build.sh --versions "4.10 6.13" --targets "mipsel" --release
• built temporary Penguin image overlaying the rebuilt mipsel driver modules onto rehosting/penguin:driver-0.0.75-qemu-local
• timeout 10m python3 tests/unit_tests/basic_target/test.py --arch mipsel --image rehosting/penguin:devfs-dynamic-per-device-mipsel-clean