alloc-util: add strdupa_safe() + strndupa_safe() and use it everywhere

Let's define two helpers strdupa_safe() + strndupa_safe() which do the
same as their non-safe counterparts, except that they abort if called
with allocations larger than ALLOCA_MAX.

This should ensure that all our alloca() based allocations are subject
to this limit.

afaics glibc offers three alloca() based APIs: alloca() itself,
strndupa() + strdupa(). With this we have now replacements for all of
them, that take the limit into account.

src/basic/mkdir.c

@@ -126,7 +126,7 @@ int mkdir_parents_internal(const char *prefix, const char *path, mode_t mode, ui
         assert(*e == '/');
 
         /* drop the last component */
-        path = strndupa(path, e - path);
+        path = strndupa_safe(path, e - path);
         r = is_dir(path, true);
         if (r > 0)
                 return 0;