libcutils: share Windows networking code.

This CL moves Windows networking code from fastboot to libcutils so
that it can be shared with other host programs such as adb.

Not all libcutils networking functions have been implemented for
Windows, just those necessary for fastboot. In the next CL I will do
the same for adb, adding any additional required functions.

Unit tests have also been added to test the functions using a loopback
connection.

Bug: http://b/26236380.
Change-Id: Ibc51a67030fe69a04c23512eefa9d19b055c7c12