MFC r341420, r341473, r341651

r341420:
Eliminate duplicated code and struct member definitions in the handoff
of args data between gptboot/zfsboot and loader(8).

Despite what seems like a lot of changes here, there are no actual
changes in behavior, or in the data layout in the structures involved.
This is just eliminating identical code pasted into multiple locations.

In detail, the changes are...

• Move struct zfs_boot_args definition from libsa/zfs/libzfs.h to i386/common/bootargs.h because it is specific to x86 booting and the handoff between zfsboot and loader, and has no relation to the zfs library code in general.

• The geli_boot_args and zfs_boot_args structs both contain an identical set of member variables containing geli information. Extract this out to a new geli_boot_data struct, and embed it in the arg-passing structs.

• Provide new routines geli_import_boot_data() and geli_export_boot_data() that can be shared between gptboot, zfsboot, and loader instead of pasting identical code into several different .c files.

• Remove some checks for a NULL pointer that can never be true because the pointer being tested was set using pointer math (kargs + 1) and that can never result in NULL in this code.

r341473:
Fix args cross-threading between gptboot(8) and loader(8) with zfs support.

When loader(8) is built with zfs support enabled, it assumes that any extarg
data present is a zfs_boot_args struct, but if the first-stage loader was
gptboot(8) the extarg data is actually a geli_boot_args struct. Luckily,
zfsboot(8) and gptzfsboot(8) have always passed KARGS_FLAGS_ZFS along with
KARGS_FLAGS_EXTARG, so we can use KARGS_FLAGS_ZFS to decide whether the
extarg data is a zfs_boot_args struct.

To avoid similar problems in the future, gptboot(8) now passes a new
KARGS_FLAGS_GELI to indicate that extarg data is geli_boot_args. In
loader(8), if the neither KARGS_FLAGS_ZFS nor KARGS_FLAGS_GELI is set but
extarg data is present (which will be the case for gptboot compiled before
this change), we now check for the known size of the geli_boot_args struct
passed by the older versions of gptboot as a way of confirming what type of
extarg data is present.

In a semi-related tidying up, since loader's main() has already decided
what type of extarg data is present and set the global 'zargs' var
accordingly, don't repeat the check in extract_currdev, just check whether
zargs is NULL or not.

r341651:
Don't reference zfs-specific variables if LOADER_ZFS_SUPPORT is undefined
because the variables will be undefined too.