add flag for profile iuse injection support (EAPI 5)
add initial support for profile IUSE injection settings

Namely IUSE_IMPLICIT, USE_EXPAND_IMPLICIT, and USE_EXPAND_UNPREFIXED.
dynamically determine tracked ebuild attributes
add full support for IUSE_EFFECTIVE

Adds support for saving IUSE_EFFECTIVE to the vdb for EAPI 5 pkgs and
checks "use" function calls against the flags available in IUSE_EFFECTIVE.