fedabipkgdiff: Add a new --private-dso option

fedabipkgdiff: Add a new --private-dso option

This patch adds an option to make fedabipkgdiff compare shared
libraries that are private to the packages being considered.

It allows us to compare the private libxul.so shared library when
considering the firefox package, for instance.

	* doc/manuals/fedabipkgdiff.rst: Add documentation for the new
	--private-dso option.
	* tools/fedabipkgdiff (abipkgdiff): Pass the --private-dso option
	to abipkgdiff when asked by the user.
	(build_commandline_args_parser): Support the new --private-dso
	option.

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