Marshalling macros put_dataz and put_datalit.

When I wanted to append an ordinary C string to a BinarySink, without
any prefix length field or suffix terminator, I was using the idiom

  put_datapl(bs, ptrlen_from_asciz(string));

but I've finally decided that's too cumbersome, and it deserves a
shorter name. put_dataz(bs, string) now does the same thing - in fact
it's a macro expanding to exactly the above.

While I'm at it, I've also added put_datalit(), which is the same
except that it expects a C string literal (and will enforce that at
compile time, via PTRLEN_LITERAL which it calls in turn). You can use
that where possible to avoid the run-time cost of the strlen.

utils/antispoof.c

@@ -11,7 +11,7 @@ void seat_antispoof_msg(InteractionReadySeat iseat, const char *msg)
          * generated by the client, then we can just use the message
          * unmodified as an unspoofable header.
          */
-        put_datapl(sb, ptrlen_from_asciz(msg));
+        put_dataz(sb, msg);
     } else if (*msg) {
         /*
          * Otherwise, add enough padding around it that the server

utils/prompts.c

@@ -34,7 +34,7 @@ void add_prompt(prompts_t *p, char *promptstr, bool echo)
 void prompt_set_result(prompt_t *pr, const char *newstr)
 {
     strbuf_clear(pr->result);
-    put_datapl(pr->result, ptrlen_from_asciz(newstr));
+    put_dataz(pr->result, newstr);
 }
 
 const char *prompt_get_result_ref(prompt_t *pr)