Use wget instead of curl in puavo-install-and-update-ltspimages.
There was a serious problem with curl: if given "--continue-at -"
to curl and file is already done, curl appends the html error message
to the file, thus corrupting it.  This might happen in some cases,
when rdiff-file is fetched, but not yet checked with cksum.
If cksum could not finish for some reason (for example host has shut down),
the rdiff-file will get corrupted in the next run and thus is removed.
With wget this issue does not occur.

Progress information with wget is also nicer, if a file is partially
finished, it knows how to report progress correctly instead of starting
the progress information always from 0% like curl does.