tree-wide: use newa() instead of alloca() wherever we can

Typesafety is nice. And this way we can take benefit of the new size
assert() the previous commit added.
diff --git a/src/tmpfiles/tmpfiles.c b/src/tmpfiles/tmpfiles.c
index 03a9e1d..b66765b 100644
--- a/src/tmpfiles/tmpfiles.c
+++ b/src/tmpfiles/tmpfiles.c
@@ -3130,7 +3130,7 @@
                 return 0;
 
         path = a->items[0].path;
-        prefix = alloca(strlen(path) + 1);
+        prefix = newa(char, strlen(path) + 1);
         PATH_FOREACH_PREFIX(prefix, path) {
                 ItemArray *j;