cashew: patch in libcurl_http_fetcher changes from update_engine

This patches in update_engine commit
60e1415b31b58bc17017d29c8e06f6e5452fdf4e (AU: Watch for writes on write
fds only, and for reads on read fds.)

original code review is here:
http://codereview.chromium.org/4190009

This fix addresses a CPU consumption issue.

BUG=chromium-os:7974
TEST=Manual testing on device, running top

Change-Id: I673feef97fa8678ece21059c877e453b113fe033

Review URL: http://codereview.chromium.org/4116007