Move use_pool() to mempool.c and rename to mempool_enabled()

The only user is in hashmap.c, but it's a mempool thing.

src/basic/mempool.c

@@ -3,8 +3,10 @@
 #include <stdint.h>
 #include <stdlib.h>
 
+#include "env-util.h"
 #include "macro.h"
 #include "mempool.h"
+#include "process-util.h"
 #include "util.h"
 
 struct pool {
@@ -70,8 +72,19 @@ void mempool_free_tile(struct mempool *mp, void *p) {
         mp->freelist = p;
 }
 
-#if VALGRIND
+bool mempool_enabled(void) {
+        static int b = -1;
 
+        if (!is_main_thread())
+                return false;
+
+        if (b < 0)
+                b = getenv_bool("SYSTEMD_MEMPOOL") != 0;
+
+        return b;
+}
+
+#if VALGRIND
 void mempool_drop(struct mempool *mp) {
         struct pool *p = mp->first_pool;
         while (p) {
@@ -81,5 +94,4 @@ void mempool_drop(struct mempool *mp) {
                 p = n;
         }
 }
-
 #endif