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;
             }

unix/storage.c

@@ -557,7 +557,7 @@ bool enum_settings_next(settings_e *handle, strbuf *out)
     fullpath = strbuf_new();
 
     char *sessiondir = make_filename(INDEX_SESSIONDIR, NULL);
-    put_datapl(fullpath, ptrlen_from_asciz(sessiondir));
+    put_dataz(fullpath, sessiondir);
     sfree(sessiondir);
     put_byte(fullpath, '/');
 
@@ -565,7 +565,7 @@ bool enum_settings_next(settings_e *handle, strbuf *out)
 
     while ( (de = readdir(handle->dp)) != NULL ) {
         strbuf_shrink_to(fullpath, baselen);
-        put_datapl(fullpath, ptrlen_from_asciz(de->d_name));
+        put_dataz(fullpath, de->d_name);
 
         if (stat(fullpath->s, &st) < 0 || !S_ISREG(st.st_mode))
             continue;                  /* try another one */