stand/libsa: zfs use standard ZFS_EARLY stuff

Now that the minor issues preventing zfs.c from using CFLAGS_EARLY have
been fixed, use that mechanism like everything else that needs the
OpenZFS spl headers. This simplifies things somewhat. Update comments to
document why zfs.c is still special, though in different ways.

Note: We also use the fact that NEED_SOLARIS_BOOLEAN is only defined in
an environment where the solaris compat boolean stuff will be defined
prior to this point (eg, when we're building zfs.c in libsa), but not in
other environments (like when we're building mkimage and stand-alone
boot loaders that don't use libsa). These latter uses should be changed
to use the same ZFS compile env, but aren't as part of this commit.
This has to be done in the same change as the ZFS_EARLY change to not
break zfs.c building for one commit affecting bisectabiltiy.

Sponsored by: Netflix
Reviewed by: tsoome, delphij
Differential Revision: https://reviews.freebsd.org/D35894