Pull in new slots when appropriate, bug #478188.
emerge: accept --pkg-format option

Accept --pkg-format option which will override settings of
PORTAGE_BINPKG_FORMAT. Currently takes choices of 'tar' (original gentoo
binary package), 'rpm' or its combinations.

Signed-off-by: Tomáš Čech <sleep_walker@suse.cz>
Exclude PORTAGE_BINPKG_FORMAT from ebuild env.
_want_update_pkg: respect --exclude
dep_zapdeps: skip want_update_pkg if pkg in graph