MFC r304321,304753,304754,306751,316077,316110:
SHA512, skein, large block support for loader zfs
MFC r304321: Add SHA512, skein, large blocks support for loader zfs.
MFC r304753: loader: zio_checksum_verify() must test spa for NULL pointer
MFC r304754: r304321 broken bhyve zvol VM bhyveload hang 100% WCPU
MFC r306751: Disable loop unrolling in skein for sys/boot
MFC r316077: Unbreak compilation with gcc 4.2.1
MFC r316110: Use -Wno-missing-declarations with CWARNFLAGS for skein.c