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

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