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/fs-util.c

@@ -56,7 +56,7 @@ int rmdir_parents(const char *path, const char *stop) {
         if (!path_is_safe(stop))
                 return -EINVAL;
 
-        p = strdupa(path);
+        p = strdupa_safe(path);
 
         for (;;) {
                 char *slash = NULL;