Add --harmless option to abipkgdiff

Add --harmless option to abipkgdiff

This option that is present for the abidiff tool was missing for
abipkgdiff.

	* doc/manuals/abidiff.rst: Fix a typo.
	* doc/manuals/abipkgdiff.rst: Document the --harmless option.
	* tools/abipkgdiff.cc: Update copyright year.
	(options::show_harmless_changes): Add new data member.
	(options::options): Initialize the new data member.
	(display_usage): Add a help string for the new --harmless option.
	(parse_command_line): Parse the new --harmless option.
	(set_diff_context_from_opts): Configure the diff context
	accordingly, if the user provided the --harmless option.

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