Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Create boot_img as a global variable

Get the information from the image that we're booting and store it in
a global variable. Prefer using this to passing it around. Remove the
special case for zfs that set the preferred boot handle by having it
uses this global variable diretly.

Reviewed by: kevans@
Differential Revision: https://reviews.freebsd.org/D20015

Start to reduce the number of #ifdef EFI_ZFS_BOOT

There's a number of EFI_ZFS_BOOT #ifdefs that aren't needed, or can be
eliminated with some trivial #defines. Remove the EFI_ZFS_BOOT ifdefs
that aren't needed. Replace libzfs.h include which is not safe to
include without EFI_ZFS_BOOT with efizfs.h which is and now
conditionally included libzfs.h. Define efizfs_set_preferred away
and define efi_zfs_probe to NULL when ZFS is compiled out.

Provide a more direct interface to tell ZFS what the preferred handle
is. We tell the ZFS code now, and it checks rather than having a
callback to do the checks.

This will allow us to have a more graceful fallback code. In the
future, it's anticipated that we may fallback to a more global search
(or implement a command to do so) when reqeusted by the user, or we
detect a violation of the UEFI Boot Manager protocol severe enough to
warrant this backstop. For now, it just allows us to get rid of img as
a global.

Sponsored by: Netflix

Refactor currdev setting

Refactor the currdev setting to find the device we booted from. Limit
searching when we don't already have a reasonable currdev from that to
the same device only. Search a little harder for ZFS volumes as that's
needed for loader.efi to live on an ESP.

Sponsored by: Netflix
Differential Review: https://reviews.freebsd.org/D13784

Move sys/boot to stand. Fix all references to new location

Sponsored by: Netflix