(var.h, plague.h): Since all that was left in var.h were some plague

defines, import these defines into plague.h, drop var.h and include
plague.h where appropriate.
Remove some 'register' keywords at the same time.
No functional changes.

src/lib/commands/expl.c

@@ -33,7 +33,7 @@
 
 #include "misc.h"
 #include "player.h"
-#include "var.h"
+#include "plague.h"
 #include "sect.h"
 #include "xy.h"
 #include "nsc.h"
@@ -48,7 +48,7 @@ static int explore_map(s_char *what, coord curx, coord cury, s_char *arg);
 int
 explore(void)
 {
-    register int amount;
+    int amount;
     struct sctstr sect;
     struct sctstr endsect;
     struct sctstr start;