curl: set a 100K buffer size by default

Test command 'time curl http://localhost/80GB -so /dev/null' on a Debian
Linux.

Before (middle performing run out 9):

 real    0m28.078s
 user    0m11.240s
 sys     0m12.876s

After (middle performing run out 9)

 real    0m26.356s (93.9%)
 user    0m5.324s  (47.4%)
 sys     0m8.368s  (65.0%)

Also, doing SFTP over a 200 millsecond latency link is now about 6 times
faster.

Closes #1446