webserver: use pthreads to handle multiple clients

webserver: use pthreads to handle multiple clients

Clients like browsers prefer to open many connections and keep them open
for a while, so that pages with lot of subelements would take a while to
load (if at all), by using threads as all servers do some way or another
we can resolve this. libapt is not intended to be pthread-safe and stuff
like the storage of the last return code doesn't make too much sense if
multiple clients interact with us, but it is good enough for now and an
other interesting (mis)use of libapt in general.

Git-Dch: Ignore