Martin Kühne authored Oct 17, 2016 and Allan McRae committed Dec 05, 2016

In FS#43434, Downloads which fail and are restarted on a different server
will resume and may display a negative download speed. The payload's progress
in libalpm was not properly reset which ultimately caused terminal noise
because the line width calculation assumes positive download speeds.

This patch fixes the incomplete reset of the payload by mimicing what
be_sync.c:alpm_db_update() does over in sync.c:download_single_file().
The new dload.c:_alpm_dload_payload_reset_for_retry() extends beyond the
current behavior by updating initial_size and prevprogress for this case.
This makes pacman reset the progress properly in the next invocation of the
callback and display positive download speeds.

Fixes FS#43434.

Signed-off-by: Martin Kühne <mysatyre@gmail.com>
Signed-off-by: Allan McRae <allan@archlinux.org>