libcutils: add multi-buffer socket send.

Unix and Windows both have functions to write multiple buffers to a
socket with a single call but they have very different signatures. This
CL creates some cross-platform functions to be able to perform these
operations in a uniform way, which will be required for upcoming
fastboot functionality.

This CL also fixes some inconsistent spacing in the touched files.

Bug: http://b/26558551
Change-Id: I8f14d52d3a1de1f3b464267666d6cd3b54263238