Changed options to emerge for ebuild.update

Added options --newuse and --oneshot to emerge command in ebuild.update.
The --newuse option will detect USE flag changes in packages, allowing a
package to be reinstalled at the same version but updated with new USE
flags. The --oneshot will prevent the package from being recorded in the
world file. Gentoo maintains a "world" set that is all the packages that
were explicitly installed. If the user is updating a dependancy, they
would not want the package added to the world file, if they are updating
a package already in the world file, it will remain there.