Header consolidation - main.h, misc.h, and share.h merge to advent.h.

For a project this small size with a small type ontology, it's better
C style to have one header where all the assumptions about types and
structures are immediately visible.

This is a first step towards turning all that global state into a handful
of structures - probably just two, one saveable as a binary game state.

score.c

@@ -1,7 +1,5 @@
 #include <stdlib.h>
-#include "misc.h"
-#include "main.h"
-#include "share.h"
+#include "advent.h"
 #include "database.h"
 
 /*