and trivial fix for comment in conf

We allow a password to be set in /etc/update-extlinux.conf. Instructions for
generating this are in /etc/update-extlinux.conf. For reference, here is another
(equivalent) way to generate the MD5 password: openssl passwd -1 -salt yy pass

If one sets a password, one will presumably want to make
/etc/update-extlinux.conf world-unreadable. We don't do that for you; however
we do make sure when a password is present to make the /boot/extlinux.conf
files we generate be world-unreadable.

Of the auto-generated entries, only HDT (if this is generated) is now
configured to respect the password; however, you can include "MENU PASSWD" in
any entries you put in /etc/update-extlinux.d/.

For example, I configure my BIOS to only boot from the internal drive, but I
have an entry in /etc/update-extlinux.d that permits chain-booting from a USB
key, and I have this entry configured to also require the password. (The BIOS
is also passworded, so that these settings can't be changed willy-nilly.)

Conflicts:
	main/syslinux/update-extlinux.conf

HDT provides a curses-like interface to display lots of hardware info
about your machine at bootloader time.

We don't install /boot/hdt.c32, but if it's present (it can be copied from
/usr/share/syslinux/hdt.c32), we add a menu entry for it---in preference to,
rather than in addition to, memtest, since HDT has a menu entry which invokes
memtest.

Using HDT to its full capacity requires finding or generating modules.pcimap
and pci.ids files for your machine, and installing them in /boot. We might want
to document this, which I don't here (but the online docs for HDT do). These
aren't required to use other functionality of HDT; and it's pretty useful
already without those.

We don't install /boot/reboot.c32, but if it's present (it can be copied from
/usr/share/syslinux/reboot.c32), we add a menu entry for it---just like
with memtest.

Add a comment to /etc/update-extlinux.conf stating this.

It'd already be possible to get a reboot entry using the
/etc/update-extlinux.d/ folder, but this patch provides a more intelligent
framework with nicer layout.

The syslinux sourceball also provides a shutdown module, but I couldn't get this to
work and looking at the sources reveals it to be for machines with APM enabled.
Not sure how many machines that applies to anymore.

Add a blank line between any entries added from /etc/update-extlinux.d/*.

Move the memtest entry (if any) after a separator line, below these.

If xen.gz is present, there are multiple kernels, and later ones lack an
initramfs, they'll currently be (wrongly) configured to use a initramfs anyway.
Fixed.

We also refactor the check for /boot/initramfs-$tag, and configure this using a
separate INITRD line, instead of adding it to the APPEND line. In passing, this
also closes a second might-use-a-stale-initramfs bug.