sort: let --check take precedence over --merge

[emirmrkaljevic]

Summary

When both --check (-c) and --merge (-m) are given with a single input, GNU sort still checks that the input is ordered. uutils preferred the merge path, printed the file, and exited 0 even when the input was disordered.

$ printf 'b\na\n' > unsorted.txt
$ sort -c -m unsorted.txt          # GNU → exit 1, disorder diagnostic
$ ./sort -c -m unsorted.txt        # uutils before → printed b\\na, exit 0

exec() now runs the check path when settings.check is set, matching GNU. Multiple-file -c rejection is unchanged.

Closes #15380

Test plan

• cargo test --features "feat_common_core sort" --no-default-features test_check_with_merge_still_checks — pass
• Manual: printf 'b\na\n' | sort -c -m → exit 1 / disorder; sorted input → exit 0
• Recorded GNU case replayed with behave against the fixed binary — 1/1 pass

Found with behave.