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.

coccinelle/strdupa.cocci

@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+@@
+expression x;
+@@
+- strdupa(x)
++ strdupa_safe(x)
+@@
+expression x, n;
+@@
+- strndupa(x, n)
++ strndupa_safe(x, n)