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/parse-util.c

@@ -704,7 +704,7 @@ int parse_dev(const char *s, dev_t *ret) {
         if (s[n] != ':')
                 return -EINVAL;
 
-        major = strndupa(s, n);
+        major = strndupa_safe(s, n);
         r = safe_atou(major, &x);
         if (r < 0)
                 return r;
@@ -765,7 +765,7 @@ int parse_loadavg_fixed_point(const char *s, loadavg_t *ret) {
         if (!d)
                 return -EINVAL;
 
-        i_str = strndupa(s, d - s);
+        i_str = strndupa_safe(s, d - s);
         f_str = d + 1;
 
         r = safe_atolu_full(i_str, 10, &i);