Move signature payload creation to download engine

Until now callee of ALPM download functionality has been in charge of
payload creation both for the main file (e.g. *.pkg) and for the accompanied
*.sig file. One advantage of such solution is that all payloads are
independent and can be fetched in parallel thus exploiting the maximum
level of download parallelism.

To build *.sig file url we've been using a simple string concatenation:
$requested_url + ".sig". Unfortunately there are cases when it does not
work. For example an archlinux.org "Download From Mirror" link looks like
this https://www.archlinux.org/packages/core/x86_64/bash/download/ and
it gets redirected to some mirror. But if we append ".sig" to the end of
the link url and try to download it then archlinux.org returns 404 error.

To overcome this issue we need to follow redirects for the main payload
first, find the final url and only then append '.sig' suffix.
This implies 2 things:
 - the signature payload initialization need to be moved to dload.c
 as it is the place where we have access to the resolved url
 - *.sig is downloaded serially with the main payload and this reduces
 level of parallelism

Move *.sig payload creation to dload.c. Once the main payload is fetched
successfully we check if the callee asked to download the accompanied
signature. If yes - create a new payload and add it to mcurl.

*.sig payload does not use server list of the main payload and thus does
not support mirror failover. *.sig file comes from the same server as
the main payload.

Refactor event loop in curl_multi_download_internal() a bit. Instead of
relying on curl_multi_check_finished_download() to return number of new
payloads we simply rerun the loop iteration one more time to check if
there are any active downloads left.

Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com>
Signed-off-by: Allan McRae <allan@archlinux.org>

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