Add a zero-copy backend

Add a zero-copy backend

This is the initial implementation of a zero-copy backend,
also known as 'zlog' in various places.

Many facilities (logutil, stdout redirection etc) are to be
implemented in further commits, only writing is supported
so far.

It aims to minimize the cost of writing logs above all else.
When it comes to perf, it's pushin' it to the max, rollin' deep, baby.

Some traits:
 + limited syscalls (one per N logs)
 + no needless copies (writes directly to destination)
 - no way to monitor logs efficiently (which also has
   consequences for features like QoS)
 - kernel-based and somewhat convoluted

Change-Id: I183ca7ee29aaeb6d24b2c5da96fc353f2c298d3e
Signed-off-by: Michal Bloch <m.bloch@samsung.com>