fastboot: implement UDP networking interface.

This CL creates a UdpSocket class that provides a simple unified
interface to send and receive UDP packets for all platforms. Nothing
uses this interface yet except for tests.

The eventual goal is to implement a UDP protocol for fastboot, but it
makes the code much simpler and more modular if we handle the low-level
networking here independently of our custom fastboot protocol.

Some of the Windows code is similar to adb. I'd like to create a
library to hold the common functionality, but it is going to be a
little delicate to separate out the features unique to adb (e.g. the
custom file descriptor system), and I don't want to risk breaking
something in adb before the holiday break, so I'm hoping to get this in
for now and merge them early next year.

Tests are included in this CL to exercise this functionality using a
loopback connection.

Bug: http://b/26154763.
Tests: `fastboot_test` loopback tests on Linux, Mac, and Windows 7.
Change-Id: I81d1b7ace8d864246b99f6c80b8e29f64b8aa375

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