efibootmgr: provide explicit error message if run as non-root

[imp]

usr.sbin/efibootmgr/efibootmgr.c

1107

This works...

I'd also looked at machdep.bootmethod, but then discovered that's not on all UEFI-supporting systems and that's why I never finished up the patches.

[imp]

usr.sbin/efibootmgr/efibootmgr.c

1107

Though thinking about it, arm and arm64 both require UEFI (effectively), though arm64 can get its device tree from fdt.
Riscv64 might also require it, but I'm way less sure about that. It's rare enough we needn't optimize it yet.
Either way, we'd cover the vast majority of cases if we added an

#ifdef amd64
/* code here to get machdep.bootmethod and compare against UEFI and say BIOS not supported instead */
#endif
errx(1, "efi variables not supported...");