Don't internalize all but main by default.

This is mostly a cleanup, but it changes a fairly old behavior.

Every "real" LTO user was already disabling the silly internalize pass
and creating the internalize pass itself. The difference with this
patch is for "opt -std-link-opts" and the C api.

Now to get a usable behavior out of opt one doesn't need the funny
looking command line:

opt -internalize -disable-internalize -internalize-public-api-list=foo,bar -std-link-opts