Better diagnostics when abipkgdiff has an extra argument

Better diagnostics when abipkgdiff has an extra argument

When abipkgdiff is launched with three packages to compare rather than
just two, it gives a nonsensical error, just like if a wrong option was
given.  This patch fixes that.

	* tools/abipkgdiff.cc (options::wrong_arg): New data member.
	(parse_command_line): Set options::wrong_arg
	to the wrong argument passed.
	(main): Tell wrong argument case apart, and report it.

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