abidw: add support for CTF

abidw: add support for CTF

This patch adds support for CTF to the abidw utility.  It depends on
the previous patch that makes abigail::ctf_reader::read_corpus to
return a status code.

	* tools/abidw.cc: Conditionally include abg-ctf-reader.h.
	(load_corpus_and_write_abixml): Do not get a
	dwarf_reader::read_context as an argument.
	(main): Adjust call to load_corpus_and_write_abixml accordingly.
	(struct options): New option use_ctf.
	(options): ... and initialize it.
	(display_usage): Document --ctf.
	(parse_command_line): Handle --ctf.
	* doc/manuals/abidw.rst: Document --ctf.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>