tool_operate: refuse (--data or --form) and --continue-at combo

libcurl assumes that a --continue-at resumption is done to continue an
upload using the read callback and neither --data n

tool_operate: refuse (--data or --form) and --continue-at combo

libcurl assumes that a --continue-at resumption is done to continue an
upload using the read callback and neither --data nor --form use
that and thus won't do what the user wants. Whatever the user wants
with this strange combination.

Add test 426 to verify.

Reported-by: Smackd0wn on github
Fixes #11081
Closes #11083

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