partition: add GPT and MBR selection policies

[wow20220809-lab]

Summary

partition.Read currently returns GPT as soon as GPT can be read. This can
select readable residual GPT metadata even when the disk contains a valid
conventional MBR.

This change introduces two explicit selection policies:

• GPTWithMBR: prefers a valid conventional MBR while continuing to use GPT
  for protective and hybrid MBR layouts. This is the new default.
• GPTIgnoreMBR: preserves the previous GPT-first behavior.

The existing partition.Read signature remains unchanged and delegates to
ReadWithPolicy using GPTWithMBR.

Behavior

GPT	MBR layout	GPTWithMBR	GPTIgnoreMBR
Valid	No valid MBR	GPT	GPT
Valid	Protective MBR	GPT	GPT
Valid	Hybrid MBR	GPT	GPT
Valid	Conventional MBR without valid 0xEE entry	MBR	GPT
Invalid	Valid MBR	MBR	MBR
Invalid	Invalid MBR	Error	Error

A protective entry is recognized by type 0xEE with starting LBA 1. Its size
is intentionally not required to span the current disk, since a disk image may
have been copied to a larger device without updating the PMBR size.

Tests

Added coverage for:

• default GPTWithMBR behavior
• conventional MBR with readable residual GPT metadata
• GPTIgnoreMBR
• protective and hybrid MBR layouts
• missing or invalid MBR
• invalid protective entries
• invalid GPT fallback
• invalid policy values

Validation performed:

• go test ./partition/... -count=1
• go test -race ./partition/... -count=1
• go vet ./...
• golangci-lint run ./partition/...

Fixes #422

[deitch]

This is great, thank you.

[deitch]

Thanks, merged in. Who are you @wow20220809-lab ; and how do you use it?

[wow20220809-lab]

Thanks for merging it!

I'm a cloud security engineer. Detecting security risks in cloud disks is an important part of our work, and go-diskfs helps us identify disk layouts and mount those disks for comprehensive security scanning.

Thank you very much for creating and maintaining this project!