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.

(cherry picked from commit 3830659e99640001c09d26dfc0e1bbd77d919a62)
(cherry picked from commit a137f7997efc8a34da75a7643c8fade4be67e82c)
(cherry picked from commit 4583682ec14cc63f063b6769b7706af1891b0934)