HTTP: return larger than 3 digit response codes too

HTTP 1.1 is clearly specified to only allow three digit response codes,
and libcurl used sscanf("%3d") for that purpose. This made libcurl
support smaller numbers but not larger. It does now, but we will not
make any specific promises nor document this further since it is going
outside of what HTTP is.

Bug: http://curl.haxx.se/bug/view.cgi?id=1441
Reported-by: Balaji