add option to link sbin/kresd to jemalloc

And by default do so iff jemalloc is found.

I chose the simplicity of adding the chosen allocator just in the single binary. Other sbin/* don't matter really, and dynamic libs (e.g. modules) will just follow whoever loaded them.

daemon/main.c

@@ -35,6 +35,26 @@
 #endif
 #include <libknot/error.h>
 
+#if ENABLE_JEMALLOC
+/* Make the jemalloc library needed.
+ *
+ * The problem is with --as-needed for linker which is added by default by meson.
+ * If we don't use any jemalloc-specific calls, linker will decide that
+ * it is not needed and won't link it.  Making it needed seems better than
+ * trying to override the flag which might be useful in some other cases, etc.
+ *
+ * Exporting the function is a very easy way of ensuring that it's not optimized out.
+ */
+#include <jemalloc/jemalloc.h>
+KR_EXPORT void kr_jemalloc_unused(void)
+{
+	malloc_stats_print(NULL, NULL, NULL);
+}
+/* We don't use threads (or rarely in some parts), so multiple arenas don't make sense.
+   https://jemalloc.net/jemalloc.3.html
+ */
+KR_EXPORT const char *malloc_conf = "narenas:1";
+#endif
 
 struct args the_args_value;  /** Static allocation for the_args singleton. */