More sensible to bring all the pieces of per-puzzle descriptive text
together into one place, so they can be easily reused everywhere
they're needed.

install the actual games, not the auxiliary binaries or nullgame.

[originally from svn r9887]

to it giving each game's "internal" name (as seen in the source file,
.R etc) and also a brief description of the game. The idea of the
latter is that it should be usable as a comment field in .desktop
files and similar.

[originally from svn r9858]

in the 'unfinished' directory for a while, and has now been finished
up thanks to James Harvey putting in some effort and galvanising me to
put in the rest. This is 'Pearl', an implementation of Nikoli's 'Masyu'.

The code in Loopy that generates a random loop along grid edges to use
as the puzzle solution has been abstracted out into loopgen.[ch] so
that Pearl can use it for its puzzle solutions too. I've also
introduced a new utility module called 'tdq' (for 'to-do queue').

[originally from svn r9379]