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- uniso now properly checks freespace early
- it runs fdatasync, is interruptible, and few seconds faster
- no loop back mounting required

* This helps uniso to take majority of the io wait time for writing
  files to disk. And allows aborting setup-bootable when using uniso.
* Makes uniso abort earlier if disk is too low.

New features:

* auto detect EFI boot and install EFI ESP (force with env USE_EFI=1)
* allow specifying bootloader by env BOOTLOADER (grub or syslinux)
* allow specifying disk label by env DISKLABEL (dos or gpt)

TODO:

* Make sure MBR is only overwritten on native installs.
* Do some more checks to prevent non standard/supported setup
  like GPT on BIOS installs.

TESTS run on ESXi 6.5:

Simple native install

* syslinux OK
* GRUB OK

Extended native install

* LVM OK
* RAID OK
* RAID+LVM OK

EFI install

* Default OK
* LVM OK see: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=592834
* RAID NOT SUPPORTED

Data install DOS

* Default OK
* LVM OK

Data install GPT

* Default OK
* LVM OK but: /lib/rc/sh/openrc-run.sh: line 273:
  can't create /sys/fs/cgroup/openrc/syslog/tasks: nonexistent directory

they are read-only by design. this avoids a mount time warning.

ref #7524

- move extlinux_raidopt to setup_syslinux and pass bootdev instead
- pass kernel_opts to grub config too
- make sure to quote the args to setup_grub/syslinux

This patch allows to install Alpine in the disk on ppc64le.
It installs grub and also configure the 'PowerPC PReP boot'
partition correctly.

For now the installation is enabled by selecting 'sys' option (same
option used for syslinux) in the setup options.

udhcpc will print info to stderr and it confuses the password prompt

fixes #7260

Some users may know exactly which variant they want, and instead of
asking them two questions, we accept if users types in the variant at
the first question.

if the variant does not exist, then user is prompted.

- use "alpine" as hostname
- use first repo in the mirror list (normally cdn)
- don't ask about proxy

first in the list is normally a cdn

also try figure out a good video driver from lspci outout

needed for zfs

pv-grub does not support 64bit in ext4. We will likely never need a 16T
boot partition so let us just disable 64bit for /boot.

ref #6962

udhcpc will print to stdout the ip addr it got. This gets in the way for
the root password prompt so we redirect udhcpc to /dev/null

the wget -s option was removed in busybox 1.26
https://git.busybox.net/busybox/commit/?id=2972e2c4acb9c46a8cc05c857acac2b3d439bf69

ref #6683

fixes #2876

fixes #6517

The output format of util-linux losetup is different, and there's
no easy way to get uniform output from these two implementations.