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/mountpoint-util.c

@@ -156,7 +156,7 @@ static bool filename_possibly_with_slash_suffix(const char *s) {
         if (slash[strspn(slash, "/")] != 0) /* Check that the suffix consist only of one or more slashes */
                 return false;
 
-        copied = strndupa(s, slash - s);
+        copied = strndupa_safe(s, slash - s);
         return filename_is_valid(copied);
 }