globally use session-qualified imports; add Lib session

Session-qualified imports will be required for Isabelle2018 and help clarify
the structure of sessions in the build tree.

This commit mainly adds a new set of sessions for lib/, including a Lib
session that includes most theories in lib/ and a few separate sessions for
parts that have dependencies beyond CParser or are separate AFP sessions.
The group "lib" collects all lib/ sessions.

As a consequence, other theories should use lib/ theories by session name,
not by path, which in turns means spec and proof sessions should also refer
to each other by session name, not path, to avoid duplicate theory errors in
theory merges later.

Isabelle2016-1: update references to renamed constants and facts

trivial: remove some uses of find_theorems

arch_split: requalify abstract theories

arch_split: SysInitExamples checking

terminology in comments: async ep -> notifications

add arch_tcb object to C, rename aep -> ntfn

sys-init now checks

sys-init: Isabelle2015 update

sys-init: Label all limitations of the system initialiser with "LIMITATION".

sys-init: Prove the starting of threads is done correctly.

We no longer assume the starting of threads, but prove it correct
(assuming the behaviour of the scheduler).

CapDL: Made IRQ Nodes a new object type, not a small CNode.

IRQ Nodes are now their own object type in capDL. This makes it much easier
to distinguish between "real" CNodes and IRQ Nodes.

Updated:
* the capDL refinement,
* the access proofs, and
* the system initialiser.

Import release snapshot.