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.

unix/plink.c

@@ -774,7 +774,7 @@ int main(int argc, char **argv)
             while (argc > 0) {
                 if (cmdbuf->len > 0)
                     put_byte(cmdbuf, ' '); /* add space separator */
-                put_datapl(cmdbuf, ptrlen_from_asciz(p));
+                put_dataz(cmdbuf, p);
                 if (--argc > 0)
                     p = *++argv;
             }