installer: Add basic UEFI system setup.
GPT partition 0 is a 128MB FAT32 EFI system partition. Microsoft
recommends at least 100MB and FAT32 in [1]. Partition 1 is a
"DragonFly Label64" (aka "dragonfly" in gpt(8)) for the rest of
the disk. It is analogous to slice 1 in our MBR setup, and has the
same name, e.g. /dev/da0s1. The rest of the UEFI setup is identical
to an MBR setup. disklabel64, newfs*, and so forth.
We currently only support installing DragonFly to the whole disk
in UEFI mode, and we don't set any NVRAM EFI variables during
installation, so for now we are copying our stage 1 loader to
\EFI\BOOT\BOOTX64.EFI on the EFI system partition.
Also, there isn't any support for Secure Boot.
[1] https://technet.microsoft.com/en-us/library/hh824839.aspx#DiskPartitionRules