Change ACCEPT_LICENSE evaluation so that things like ACCEPT_LICENSE="* -@EULA"

Change ACCEPT_LICENSE evaluation so that things like ACCEPT_LICENSE="* -@EULA"
will work as expected. The ACCEPT_LICENSE variable is now treated as a lazily
evaluated incremental, so that * can be used to match all licenses without
every having to explicitly expand it to all licenses. It should now behave
as approved by the council in response to the "RFC: ACCEPT_LICENSE default
value (GLEP 23)" discussion:

  http://archives.gentoo.org/gentoo-dev/msg_d5c1e7285399ebc27a74bdd02cb4d037.xml

svn path=/main/trunk/; revision=13670