wget: use FEATURE_COPYBUF_KB-sized buffer. Much faster for local transfers

wget: use FEATURE_COPYBUF_KB-sized buffer. Much faster for local transfers

function                                             old     new   delta
base64enc                                              -      53     +53
gethdr                                               190     200     +10
ftpcmd                                               129     133      +4
progress_meter                                       160     122     -38
retrieve_file_data                                   431     392     -39
base64enc_512                                         46       -     -46
wget_main                                           2456    2220    -236
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 2/3 up/down: 67/-359)          Total: -292 bytes

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