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/procfs-util.c

@@ -135,7 +135,7 @@ int procfs_tasks_get_current(uint64_t *ret) {
 
         p++;
         n = strspn(p, DIGITS);
-        nr = strndupa(p, n);
+        nr = strndupa_safe(p, n);
 
         return safe_atou64(nr, ret);
 }