Fix bug 11567 updxlrator: don't prematurely release lock file

Fix bug 11567 updxlrator: don't prematurely release lock file

With Microsoft's new style of downloading updates,
where portions of a patch are requested multiple times per second,
it has become extremely common for downloads to reach > 100%.
Due to an early unlinking of the "lock" file, there is a big window of
opportunity (between the unlink and wget actually saving some data)
for multiple download/wget threads to start, adding to the same file.
So not only is bandwidth wasted by duplicate downloads running
simultaneously, but the resulting file is corrupt anyway.

The problem is noticed more often by low bandwidth users
(who need the benefits of updxlrator the most)
because then wget's latency is even longer, creating
a very wide window of opportunity.

Ultimately, this needs something like "flock", where the
file is set and tested in one operation. But for now,
settle with the current test / create lock solution, and
just stop unnecessarily releasing the lock.

Since the file already exists as a lock when wget starts,
wget now must ALWAYS run with --continue, which
works fine on a zero-sized file.

Signed-off-by: Justin Luth  <jluth@mail.com>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>