curl: add --skip-existing

With this option, the entire download is skipped if the selected target
filename already exists when the opertion is about to begin.

Test 994, 995 and

curl: add --skip-existing

With this option, the entire download is skipped if the selected target
filename already exists when the opertion is about to begin.

Test 994, 995 and 996 verify.

Ref: #11012
Closes #13993

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