wget: make "wget -O FILE URL1 URL2" concatenate output

wget: make "wget -O FILE URL1 URL2" concatenate output

Also fixes a few cases where URL1's data (like start pos)
was leaking into URL2

function                                             old     new   delta
wget_main                                           2303    2321     +18
progress_meter                                       140     152     +12
retrieve_file_data                                   397     396      -1
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 30/-1)              Total: 29 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>