[Bug-wget] merge parallel-wget into master

[Giuseppe Scrivano]

Hi,

I would like to merge the parallel-wget branch into master.  I already
merged master into parallel-wget, so it should be a no-op and make
master look identical to parallel-wget as it is now.

The idea is that the new features (threads and metalink) are not enabled
by default, but must be forced at configure time so not to break the
stable release of wget, but I don't know how much this is true as there
are many changes.

The code introduces some hacks, but I think we can deal with this for
now and refactor the code once it is on master, not much happened this
last year when we had the project on a different branch.

Is anyone against this?

-- 
Giuseppe