umem: test-load libumem in a separate namespace in the management process

libumem's atfork handler will redefine malloc() and friends so if we
load it in the manager, we cannot confine its use to the stevedore

bin/varnishd/storage/storage_umem.c

@@ -41,6 +41,7 @@
 #include <stdlib.h>
 #include <umem.h>
 #include <dlfcn.h>
+#include <link.h>
 
 #include "storage/storage.h"
 #include "storage/storage_simple.h"
@@ -262,7 +263,8 @@ smu_init(struct stevedore *parent, int ac, char * const *av)
 
 	/* Check if these load in the management process. */
 	(void) dlerror();
-	if ((libumem_hndl = dlopen("libumem.so", RTLD_NOW)) == NULL)
+	libumem_hndl = dlmopen(LM_ID_NEWLM, "libumem.so", RTLD_LAZY);
+	if (libumem_hndl == NULL)
 		ARGV_ERR("(-sumem) cannot open libumem.so: %s", dlerror());
 
 #define DLSYM_UMEM(fptr,sym)						\
@@ -282,6 +284,9 @@ smu_init(struct stevedore *parent, int ac, char * const *av)
 
 #undef DLSYM_UMEM
 
+	AZ(dlclose(libumem_hndl));
+	libumem_hndl = NULL;
+
 	e = VNUM_2bytes(av[0], &u, 0);
 	if (e != NULL)
 		ARGV_ERR("(-sumem) size \"%s\": %s\n", av[0], e);