parallel downloads of multiple debuginfo files

[Frank Ch. Eigler]

Hi -

> But once again - isn't this a problem that everyone using dwfl is going to 
> encounter? Should we not try to find a general solution to this problem and 
> fix it for all consumers of the API?

I suspect not many apps are going to have a complete list of files
they know they'll need a priori.  A live profiler tends to find out
about target files gradually.  A debugger uses debuginfo to help
enumerate other debuginfo.  DWZ files can only be even known of when
the base DWARF file is fetched.  So yeah I'm not sure there exists a
widespread general problem for which multithreading is not a workable
solution.

- FChE