join: let -o accumulate fields across occurrences like GNU

[AlejandroCoronadoN]

GNU join lets -o FORMAT appear more than once; the field specs from every occurrence accumulate in order. uutils rejects a repeated -o with a hard error:

$ join -o 1.1 -o 1.2 -o 2.2 file1 file2
join: the argument '-o <FORMAT>' cannot be used multiple times
# GNU accumulates the fields and prints 1.1 1.2 2.2

This makes several -o accumulate, matching GNU, including the auto corner cases: auto mode applies only when every -o value is exactly auto, otherwise each auto is ignored and the real specs accumulate. It uses the existing ArgAction::Append idiom (as -a and -v already do). Single--o behavior and the per-field diagnostics are unchanged.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)

[sylvestre]

Please fix the conflict

[AlejandroCoronadoN]

Fixed the conflict, rebased onto current main (the change now sits on top of the new -o caret diagnostics). Thanks!

[cakebaker]

This test is a bit unlucky as both -o auto and -o 1.1 2.2 on their own have the same output and so, when combined, you can't say which option got ignored. I would use a different format for the second option.