Use either LWP::UserAgent or curl to make HTTP requests so it
works on Windows (most Perl distros include LWP::UserAgent),
and also on OS X (whose Perl at least sometimes doesn't
include LWP::UserAgent).

Fixes #6273

R=golang-dev, alex.brainman, cldorian
CC=golang-dev
https://golang.org/cl/13330044