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/alloc-util.h

@@ -194,3 +194,19 @@ void* greedy_realloc0(void **p, size_t need, size_t size);
                 __builtin_types_compatible_p(typeof(x), typeof(&*(x))), \
                 MALLOC_SIZEOF_SAFE(x)/sizeof((x)[0]),                   \
                 VOID_0))
+
+
+/* These are like strdupa()/strndupa(), but honour ALLOCA_MAX */
+#define strdupa_safe(s)                                                 \
+        ({                                                              \
+                const char *_t = (s);                                   \
+                (char*) memdupa_suffix0(_t, strlen(_t));                \
+        })
+
+#define strndupa_safe(s, n)                                             \
+        ({                                                              \
+                const char *_t = (s);                                   \
+                (char*) memdupa_suffix0(_t, strnlen(_t, (n)));          \
+        })
+
+#include "memory-util.h"