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/socket-proxy/socket-proxyd.c

@@ -438,7 +438,8 @@ static int resolve_remote(Connection *c) {
 
         service = strrchr(arg_remote_host, ':');
         if (service) {
-                node = strndupa(arg_remote_host, service - arg_remote_host);
+                node = strndupa_safe(arg_remote_host,
+                                     service - arg_remote_host);
                 service++;
         } else {
                 node = arg_remote_host;