New inst clients

Example conversation to new inst clients.
Result is quite good according to codeclimate: https://codeclimate.com/github/yast/yast-bootloader/compare/new_inst_clients
For this kind of source-code-inside-a-string thingies I'd prefer:
```ruby
cmd = <<-eos
targetdir=#{Installation.destdir}
if test ${targetdir} = / ; then echo targetdir is / ; exit 1 ; fi
grep -E \"^[^ ]+ ${targetdir}/dev \" < /proc/mounts

if test $? = 0
then
  echo targetdir ${targetdir} already mounted.
  exit 1
else
  mkdir -vp ${targetdir}/dev
  cp --preserve=all --recursive --remove-destination /lib/udev/devices/* ${targetdir}/dev
  mount -v --bind /dev ${targetdir}/dev
fi
eos
```