layout.conf: Add manifest-required-hashes setting

layout.conf: Add manifest-required-hashes setting

Add the manifest-required-hashes setting that specifies which hashes
need to be present in the Manifest files for Portage to consider old
distfile checksums acceptable (and not refetch the files to rehash
them). It is complementary to manifest-hashes that specifies which
hashes are used in new/updated entries.

Formerly, this setting was hardcoded in Portage. However, the new
Portage release will no longer do that and instead expect layout.conf
to supply the setting.