Bug#744865: Add check and warning for unetbootin

[Steve McIntyre <steve@einval.com>]

Package: debian-installer
Severity: wishlist
Tags: d-i

As suggested in #739767...

unetbootin used to be a helpful tool for many people to create
USB-bootable installer images, but these days seems responsible for
lots of user problems and difficult-to-resolve bug reports.

If feasible, check to see if we were booted from unetbootin media. If
so, print a big warning to the user that unetbootin is potentially
problematic and not supported. (Maybe) let them continue after
acknowledging the warning anyway(?), but be sure to log the warning in
the syslog for the benefit of d-i developers.

-- System Information:
Debian Release: 7.4
  APT prefers stable
  APT policy: (500, 'stable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 3.2.0-4-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash