loader: create single zfs nextboot implementation

We should have nextboot feature implemented in libsa zfs code.
To get there, I have created zfs_nextboot() implementation based on
two sources, our current simple textual string based approach with added
structured boot label PAD structure from OpenZFS.

Secondly, all nvlist details are moved to separate source file and
restructured a bit. This is done to provide base support to add nvlist
add/update feature in followup updates.

And finally, the zfsboot/gptzfsboot disk access functions are swapped to use
libi386 and libsa.

Sponsored by: Netflix, Klara Inc.
Differential Revision: https://reviews.freebsd.org/D25324