Create Live Media should handle split ISOs

[probonopd]

We are now splitting ISOs when they become larger than 2 GB. The Create Live Media utility should be able to download and write such ISOs.

Manual workaround:

% sudo umount /dev/da0*
% cat hello-0.5.0_0E63-FreeBSD-13.0-amd64.iso.part1 >> hello-0.5.0_0E63-FreeBSD-13.0-amd64.iso.part0
% sudo dd if=hello-0.5.0_0E63-FreeBSD-13.0-amd64.iso.part0 of=/dev/da0 bs=4m status=progress

Possible approach:

import urllib.request
import shutil

self.current_offset = 0 # Increment this after each part that is being written

filename = archive[i]
with urllib.request.urlopen(url) as response, open(filename, 'wb') as out_file:
    fileObject.seek(self.current_offset, 0)
    shutil.copyfileobj(response, out_file) 

As a side effect of using shutil.copyfileobj we should also support writing local ISO files and local split ISO files like we currently do for Other... ISO URLs.