CLEANUP: pools: rename all pool functions and pointers to remove this "2"

During the migration to the second version of the pools, the new
functions and pool pointers were all called "pool_something2()" and
"pool2_something". Now there's no more pool v1 code and it's a real
pain to still have to deal with this. Let's clean this up now by
removing the "2" everywhere, and by renaming the pool heads
"pool_head_something".
diff --git a/src/vars.c b/src/vars.c
index d7e7157..c8aa5ac 100644
--- a/src/vars.c
+++ b/src/vars.c
@@ -103,7 +103,7 @@
 		size += var->data.u.meth.str.len;
 	}
 	LIST_DEL(&var->l);
-	pool_free2(var_pool, var);
+	pool_free(var_pool, var);
 	size += sizeof(struct var);
 	return size;
 }
@@ -359,7 +359,7 @@
 			return 0;
 
 		/* Create new entry. */
-		var = pool_alloc2(var_pool);
+		var = pool_alloc(var_pool);
 		if (!var)
 			return 0;
 		LIST_ADDQ(&vars->head, &var->l);