tree-wide: use reallocarray instead of our home-grown realloc_multiply (#8279)

There isn't much difference, but in general we prefer to use the standard
functions. glibc provides reallocarray since version 2.26.

I moved explicit_bzero is configure test to the bottom, so that the two stdlib
functions are at the bottom.

src/basic/alloc-util.h

@@ -74,12 +74,14 @@ _malloc_  _alloc_(1, 2) static inline void *malloc_multiply(size_t size, size_t
         return malloc(size * need);
 }
 
-_alloc_(2, 3) static inline void *realloc_multiply(void *p, size_t size, size_t need) {
+#if !HAVE_REALLOCARRAY
+_alloc_(2, 3) static inline void *reallocarray(void *p, size_t need, size_t size) {
         if (size_multiply_overflow(size, need))
                 return NULL;
 
         return realloc(p, size * need);
 }
+#endif
 
 _alloc_(2, 3) static inline void *memdup_multiply(const void *p, size_t size, size_t need) {
         if (size_multiply_overflow(size, need))

src/basic/strv.c

@@ -407,7 +407,7 @@ int strv_push(char ***l, char *value) {
         if (m < n)
                 return -ENOMEM;
 
-        c = realloc_multiply(*l, sizeof(char*), m);
+        c = reallocarray(*l, m, sizeof(char*));
         if (!c)
                 return -ENOMEM;
 
@@ -432,7 +432,7 @@ int strv_push_pair(char ***l, char *a, char *b) {
         if (m < n)
                 return -ENOMEM;
 
-        c = realloc_multiply(*l, sizeof(char*), m);
+        c = reallocarray(*l, m, sizeof(char*));
         if (!c)
                 return -ENOMEM;
 
@@ -546,7 +546,7 @@ int strv_extend_front(char ***l, const char *value) {
         if (!v)
                 return -ENOMEM;
 
-        c = realloc_multiply(*l, sizeof(char*), m);
+        c = reallocarray(*l, m, sizeof(char*));
         if (!c) {
                 free(v);
                 return -ENOMEM;