Change curl and wget commands to resume downloads

[bturmann]

Partially fixes: k0sproject/k0sctl#1131
This is the first part handling the commands curl and wget. There will be another PR regarding Windows Powershell.

Currently, repeated executions will download a complete file each time even if the file is already existing.

This change implements resuming downloads by default for curl and wget. In case, the source server does not support resuming, then this option is simply ignored and the download happens anyway.

curl
Use option -C or --continue-at <offset> to resume downloads. It is required to specify the offset which can be generalized by adding the option with -C -.

wget
Use option -c or --continue to resume downloads.

[bturmann]

I don't see the exact reason of the failed check, only "Error:" is shown.

[Copilot]

Pull request overview

Adds resumable POSIX downloads using curl and wget.

Changes:

• Adds curl continuation support.
• Enables wget’s continue option.

Suppressed comments (2)

remotefs/posixfs.go:405

• The promised fallback for servers without range support is missing. Curl documents that a -C resume attempt fails when the server does not support byte ranges; this code immediately returns that error instead of retrying a normal download, contradicting the PR description that the option is ignored and the download proceeds. Handle resume rejection by retrying without -C (while preserving genuine download errors).

		if err := s.Exec(sh.Command("curl", "-C -", "-sSLf", "-o", dst, "--", url)); err != nil {

remotefs/posixfs.go:411

• The existing TestPosixDownloadURL uses only HasPrefix("curl")/HasPrefix("wget"), so it does not verify either new resume option and already misses the malformed curl argument. Add exact command assertions for both branches, including separation of -C and -.

		if err := s.Exec(sh.Command("wget", "-c", "-qO", dst, "--", url)); err != nil {

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