Review D58990: freebsd-update: automatically update EFI and BIOS bootloaders on install

I've submitted a patch to automatically update the EFI bootloader on the
ESP during freebsd-update install:

https://reviews.freebsd.org/D58990

The problem: freebsd-update updates /boot/loader.efi on the root
filesystem, but the UEFI firmware boots from loader.efi on the EFI System
Partition — a separate FAT32 partition that freebsd-update never touches.
After a major version upgrade the ESP loader can be left several versions
stale. Since 14.1 removed the lua_path==nil compatibility shim from
core.lua, this mismatch produces a hard boot failure. If the user also ran
zpool upgrade, the system is completely unbootable without external media.

The patch hooks efi_bootloader_update.sh into install_run() in
freebsd-update.sh. It auto-detects the ESP via gpart, fingerprints
/EFI/BOOT/BOOTx64.efi before touching it (multi-OS safe), creates
/EFI/FreeBSD/loader.efi if absent, updates NVRAM, and handles BIOS GPT
bootcode as well. Opt-out via UpdateBootloader=no in freebsd-update.conf.

Tested on FreeBSD 14.0-RELEASE-p11 amd64 (UEFI, ZFS, NVMe). 197
unit/integration/error tests available at:

https://github.com/RZA-SF/freebsd-patch-for-bug279829

This is complementary to Warner Losh's D45890 (loader version-check
warnings). Feedback welcome.

Best,
Ron