libbrillo: Add curl_multi_perform, curl_multi_wait wrappers.

These methods allow polling of multi handle when there is not a managed
socket in the application driving the handle. This is useful for
transfering memory only payloads.

BUG: None
TEST=emerge

Change-Id: I2be2de7ae3bd5dbfd81a1eccbc3da60a19e7a3a6
3 files changed