Introduce a mechanism in this source tree for building the container

web pages for the Java applets. Previously, those have all been
maintained by hand in my website's svn area, which is a bit silly. Now
we have a file per puzzle in the 'html' subdirectory which contains
the puzzle's name, one or two attributes, and the instructions snippet
to go below the puzzle applet; and then there's a Perl script that
builds all the real web pages out of that by adding in the parts
common across all files: the header, footer, and middle fragment with
the <applet> tag and resizing bits and pieces.

One piece _not_ checked in here is the footer text specific to my
hosting at chiark, which I think does still belong in the www area. So
Buildscr doesn't actually build the web pages; it just delivers the
bits and pieces by which my nightly snapshot script will be able to
run the program that _does_ build them, passing that footer as an
extra argument.

[originally from svn r9780]

html/undead.html

@@ -0,0 +1,19 @@
+Undead
+<p>
+Fill in every grid square which doesn't contain a mirror with either a
+ghost, a vampire, or a zombie. The numbers round the grid edges show
+how many monsters must be visible along your line of sight if you look
+directly into the grid from that position, along a row or column.
+Zombies are always visible; ghosts are only visible when reflected in
+at least one mirror; vampires are only visible when not reflected in
+any mirror.
+
+<p>
+To place a monster, click in a square to select it, then type the
+monster's letter on the keyboard: G for a ghost, V for a vampire or Z
+for a zombie. To erase a monster, click to select a square and then
+press Backspace.
+<p>
+Right-click in a square and then type a letter to add or remove the
+monster as a pencil mark, indicating monsters that you think
+<em>might</em> go in that square.