Add libsockdriver: a library for socket drivers

This library provides abstractions for socket drivers, and should be
used as the basis for all socket driver implementations. It provides
the followi

Add libsockdriver: a library for socket drivers

This library provides abstractions for socket drivers, and should be
used as the basis for all socket driver implementations. It provides
the following functionality:

- a function call table abstraction, hiding the details of the
socket driver protocol with simple parameters and presenting the
socket driver with callback functions very similar to the BSD
socket API calls made from userland;
- abstracting data structures and helper functions for suspending
and resuming blocking calls;
- abstracting data structures and helper functions for copying data
from and to the caller.

Overall, the library is similar to lib{block,char,fs,input,net}driver
in concept. Some of the abstractions provided here should in fact be
applied to libchardriver as well. As always, for the case that the
provided message loop is too restrictive, a set of more low-level
message processing functions is provided.

Change-Id: I79ec215f5e195c3b0197e223636f987d3755fb13

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