Support up to two --wp options for abipkgdiff

Support up to two --wp options for abipkgdiff

The user should be able to specify one white list package per kernel
package on the command line.  That means the user should be able to
say:

    --wp whitelist-pkg1 --wp whitelist-pkg2

on the command line.

This patch adds support for that.

	* doc/manuals/abipkgdiff.rst: Update the documentation to say that
	--wp can be provided twice, but not more than that.
	* tools/abipkgdiff.cc (options::kabi_whitelist_packages): Rename
	kabi_whitelist_package to this, and make be of vector<string>
	type.
	(package::erase_extraction_directories): Erase the white list
	package extracted data.
	(maybe_handle_kabi_whitelist_pkg, parse_command_line): Adjust.
	(main): Make sure there is no more than 2 --wp on the command
	line.  Associate a white list package to each kernel package on
	the command line.

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