dd: fix conv=block/unblock conflict handling

[ChihweiLHBird]

Summary

conv=block and conv=unblock convert between newline-terminated lines and fixed-size records; cbs=N sets the record size. The ascii conversion implies unblock, and ebcdic and ibm imply block.

Three things were off in how uutils handled the block/unblock pair:

1. Implied directions were not considered. conv=ascii,block cbs=1 asks for both unblock (via ascii) and block, but uutils accepted it silently. GNU rejects it as a block/unblock conflict.
2. It was checked last. GNU checks conflicts in a fixed order: ascii/ebcdic/ibm, then block/unblock, then lcase/ucase, then excl/nocreat. uutils checked block/unblock after the others, so conv=block,unblock,lcase,ucase cbs=1 reported the lcase/ucase conflict instead of the block/unblock one.
3. Missing cbs was an error. GNU ignores block and unblock when cbs is not given; uutils refused with its own "specified without cbs=N" message. POSIX leaves this case unspecified, so this part is a compatibility choice rather than a bug fix.

This PR moves the block/unblock check to its place in that order, includes the implied directions, and only performs it when cbs is given.

Example

$ dd conv=ascii,block cbs=1 if=/dev/null

Before: silent, exit 0. After, same as GNU: dd: cannot combine block and unblock, exit 1.

$ dd conv=block,unblock,lcase,ucase cbs=1 if=/dev/null

Before: dd: cannot combine lcase and ucase. After, same as GNU: dd: cannot combine block and unblock.

$ printf 'ab\n' | dd conv=block status=none

Before: dd: conv=block or conv=unblock specified without cbs=N, exit 1. After, same as GNU: prints ab, exit 0.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)

[Copilot]

🟢 Approval recommended

The parser changes are coherent, correctly ordered, and covered by targeted tests.

0 open findings

What changed in this PR

Updates dd argument validation to match GNU-compatible block/unblock behavior.

Changes:

• Detects implied and explicit conversion conflicts in the correct order.
• Ignores block/unblock when cbs is absent.
• Adds regression coverage and removes the obsolete error translation.

File	Description
src/​uu/​dd/​src/​parseargs.rs	Revises conversion validation and mode selection.
src/​uu/​dd/​src/​parseargs/​unit_tests.rs	Updates the conflict unit test.
tests/​by-util/​test_dd.rs	Adds compatibility regression tests.
src/​uu/​dd/​locales/​en-US.ftl	Removes the obsolete English error.
src/​uu/​dd/​locales/​fr-FR.ftl	Removes the obsolete French error.

🧠 Review effort: Balanced

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[Copilot]

🟢 Approval recommended

The focused implementation correctly handles conflict ordering, implied directions, and missing cbs, with adequate regression coverage.

0 open findings

🧠 Review effort: Balanced

[Copilot]

🟢 Approval recommended

The focused implementation is consistent with the described behavior and has comprehensive regression coverage.

0 open findings

🧠 Review effort: Balanced

[sylvestre]

Thanks for your PR