Add a --rebuilt-binaries[=n] option, causing automatic replacement of

Add a --rebuilt-binaries[=n] option, causing automatic replacement of
installed packages with binary packages that have been rebuilt. Rebuilds
are detected by comparison of BUILD_TIME package metadata. This option is
enabled automatically when using binary packages, so rebuilt binaries are
installed with a user's typical update command. This isn't possible with
the existing @rebuild-binaries package set since that only works with
--selective=n and therefore can't be used with a typical world update.
The package set framework should support this type of behavior sometime
in the future.

svn path=/main/trunk/; revision=15364