GLEP 59: use manifest-hashes list in layout.conf

GLEP 59: use manifest-hashes list in layout.conf

The manifest-hashes layout.conf setting simply overrides the hashes
that are generated. When unspecified, the MANIFEST2_HASH_DEFAULTS
constant is used instead (current value contains SHA1, SHA256, and
RMD160).

In order to enable SHA512 and WHIRLPOOL, and drop RMD160 and SHA1,
simply set "manifest-hashes = SHA256 SHA512 WHIRLPOOL" in layout.conf.
As with the previous boolean flags approach, manifest-hashes settings
become redundant and can be removed from layout.conf after a version
of portage with equivalent defaults is stable.