Make it even harder to use uninitialized `libzfs_handle' (and
make the interface a bit simpler).
* include/grub/util/misc.h (grub_util_init_libzfs)
(libzfs_handle): Remove.
(grub_get_libzfs_handle): New prototype.
* util/misc.c [HAVE_LIBZFS] (libzfs_handle): Add `static'
attribute.
(grub_util_init_libzfs): Remove.
(grub_get_libzfs_handle): New function.
* kern/emu/getroot.c (find_root_device_from_libzfs): Use
grub_get_libzfs_handle() to obtain a libzfs handle instead of
accessing `libzfs_handle' directly.