emerge: add 3 new options similar to --exclude

emerge: add 3 new options similar to --exclude

Add --nousepkg-atoms, --useoldpkg-atoms, and --reinstall-atoms flag
to Portage

reinstall-atoms accepts a space separated list of package names or
slot atoms. Emerge will treat matching packages as if they are not
installed, and reinstall them if necessary.

useoldpkg-atoms accepts a space separated list of package names or
slot atoms. Emerge will prefer matching binary packages over newer
unbuilt packages. This is useful in case you want to request that a
particular package won't be rebuilt from source.

nousepkg-atoms accepts a space separated list of package names or
slot atoms. Emerge will ignore matching binary packages.

Change-Id: I0d73039c6a4cd63695b28ffc80215628e0e05c95

BUG=chromium-os:12507 TEST=Try out the flag

Review URL: http://codereview.chromium.org/6577024