cksum: Update error and flags handling to improver GNU's match

[RenjiSann]

This PR does 4 things:

• Move the printing of improperly formatted inline warning to the process_checksum_line loop instead of only printing in the case of a problem of algorithm parsing (necessary for passing tests for --warn)
• Introduce another rather shitty variable for "caching" the algorithm that was used in the previous checksum line, so we can use it to print a warning message for a potentially incorrect line after.

Example:

CHECKSUM.txt:
  SM3 (foo.txt) = xxxxxxxxxxx
  an invalid line

cksum -c CHECKSUM.txt --warn
foo.txt: OK
CHECKSUM.txt: 2: improperly formatted SM3 checksum line      <-- SM3 is from the previous line

• Change the way the --status, --quiet and --warn flags interact together. In GNU, these flags supersedes each other to set a specific verbose level, while our implementation was just setting flags. Now, our implementation is using a verbose logic as well (STATUS, QUIET, NORMAL, WARN), and the last flag passed on CLi will override the others.

• Fix a bug which prevented cksum to return with an error exit code upon a failing checksum in case --ignore-missing was passed.

In addition to this, I have ported the gnu test cksum/cksum-c.sh directly in the rust testsuite

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/cksum/cksum-c is no longer failing!
Congrats! The gnu test tests/misc/stdbuf is no longer failing!

[RenjiSann]

Force pushed style fixes (diff)

[sylvestre]

this is great and fun :)

[sylvestre]

maybe create a function for this ?

[RenjiSann]

Indeed ! Done 👍

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/cksum/cksum-c is no longer failing!
Congrats! The gnu test tests/misc/stdbuf is no longer failing!

[RenjiSann]

Move ChecksumVerrbose variable creation to a function (diff)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/cksum/cksum-c is no longer failing!
Congrats! The gnu test tests/misc/stdbuf is no longer failing!

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/cksum/cksum-c is no longer failing!
Skipping an intermittent issue tests/misc/usage_vs_getopt (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/cksum/cksum-c is no longer failing!
Skipping an intermittent issue tests/misc/usage_vs_getopt (passes in this run but fails in the 'main' branch)

[RenjiSann]

@sylvestre ready for review when you have some time 👍

[sylvestre]

thanks :)

[RenjiSann]

And that's 100% passed GNU tests for cksum 🥳

[cakebaker]

@RenjiSann well done :)