boot/zfs: Extend zfsimpl.h and make it easier to use

Some makefs(8) patches make use of zfsimpl.h (not zfsimpl.c though) to
provide definitions for various on-disk structures. Most of this diff
simply adds new definitions that are useful.

Also reduce dependencies of the header:

• remove an unused list_node_t field to drop the sys/list.h dependency
• replace CTASSERT with _Static_assert

And fix the declaration of decode_embedded_bp_compressed().

No functional change intended.

Reviewed by: tsoome
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35278