Add function (ShellWget URL OUTFILE) that calls the shell to execute 'wget URL -O OUTFILE'

[fghalasz]

This PR adds a function (ShellWget URL OUTFILE) that executes wget URL -O OUTFILE in a separate Unix shell. URL is a string containing an http(s):// url representing a file on the web. That file will be downloaded from the web and placed in OUTFILE (which is a versioned {DSK} file). No format or other checking is done on the URL before it is handed to wget.

ShellWget will ERROR out if wget is not available on the current machine.

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[pamoroso]

It works well but, if ShellWget is called from an Exec, the download pauses for the user to press a key when the page is full. What about running wget with the -q switch or another option that generates less output?