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.

misc.c

@@ -3,9 +3,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <sys/time.h>
-#include "main.h"
-#include "share.h"	/* for SETUP */
-#include "misc.h"
+#include "advent.h"
 #include "funcs.h"
 #include "database.h"