Initial support of the serialization of the KMI of a Linux Kernel Tree

Initial support of the serialization of the KMI of a Linux Kernel Tree

We have the kmidiff program that takes two Linux Kernel trees
containing the vmlinux binary and its modules and compare their Kernel
Module Interface, aka KMI.

We need to be able to serialize (save in a file) a representation of
that KMI.  We need to load that KMI back, and compare two serialized
KMIs.

This patch implements the serialization of the KMI of a Linux Kernel
tree.  It actually serializes an instance of abigail::ir::corpus_group
that is a collection of instances of abigail::ir::corpus.  The KMI of
each individual binary (vmlinux or kernel module) is represented by
one abigail::ir::corpus.  All the corpora share the same definitions
of types and decls, whenever that makes sense.

The patch thus factorizes the routines used to walk a Linux kernel out
of the kmidiff program.  These routines are then re-used in the abidw
program to make it walk a Linux kernel tree (when the --linux-tree
option is provided), load the vmlinux and module binaries as an
instance of abigail::corpus_group and serialize it out into an output
stream.

	* include/abg-tools-utils.h (check_dir)
	(get_binary_paths_from_kernel_dist)
	(build_corpus_group_from_kernel_dist_under): Declare new
	functions.  The last two functions are being moved from
	tools/kmidiff.cc so that they can be re-used.
	* include/abg-writer.h (write_corpus): Declare one overload that
	takes a write_context parameter.
	(write_corpus_group): Declare three overloads of this new function.
	* src/abg-tools-utils.cc (check_dir): Define new function.
	(load_generate_apply_suppressions, is_vmlinux, is_kernel_module)
	(find_vmlinux_and_module_paths)
	(get_binary_paths_from_kernel_dist)
	(build_corpus_group_from_kernel_dist_under): Define new functions.
	* src/abg-writer.cc (write_context::set_annotate): Define new
	member function.
	(write_corpus): Add an overload that takes a write_context.  Adapt
	the existing overload to make it use this new one.
	(write_corpus_group): Define this new function and two additional
	overloads for it.
	* tools/kmidiff.cc (set_suppressions, is_vmlinux)
	(is_kernel_module, find_vmlinux_and_module_paths)
	(get_binary_paths_from_kernel_dist)
	(build_corpus_group_from_kernel_dist_under): Remove.
	(main): Adjust the call to
	build_corpus_group_from_kernel_dist_under as its arguments are now
	adapted since it's been factorized out into abg-tools-utils.h.
	* tools/abidw.cc (options::corpus_group_for_linux): Define new
	data member.
	(options::options): Adjust.
	(display_usage): Add help strings for the new --linux-tree option.
	(load_corpus_and_write_abixml): Factorize this function out of the
	main function.
	(load_kernel_corpus_group_and_write_abixml): Define new function.
	(main): Use the factorized load_corpus_and_write_abixml and the
	new load_corpus_and_write_abixml functions.
	* tests/test-read-write.cc: Adjust.
	* doc/manuals/abidw.rst: Add documentation for the new
	--linux-tree option.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>