fix: detect V4L2 capture devices by capability bits

[taigrr]

Summary

• detect capture devices by checking V4L2 capability bits instead of matching one exact value
• respect the capabilities fallback when V4L2_CAP_DEVICE_CAPS is absent
• close opened device handles and add regression tests for the capability parsing logic

Testing

• go test ./...
• go test -race ./...
• go vet ./...
• staticcheck ./...