Unfortunately, some sites do not support the HTTP/1.1 range header and
just serve the whole file, which takes it's time and may result in a
timeout.
Fixes#2819
We call curl a second time if the first check fails. This gives download
sites time to reconsider their response and helps, for example, to check
the qemu-usb port.
The 'check_port_source' checks whether all remote sources defined for a given
port are currently available. It returns zero, when all remote resources are
available.
Fix#1430
