Generalise strbuf_catf() into put_fmt().

marshal.h now provides a macro put_fmt() which allows you to write
arbitrary printf-formatted data to an arbitrary BinarySink.

We already had this facility for strbufs in particular, in the form of
strbuf_catf(). That was able to take advantage of knowing the inner
structure of a strbuf to minimise memory allocation (it would snprintf
directly into the strbuf's existing buffer if possible). For a general
black-box BinarySink we can't do that, so instead we dupvprintf into a
temporary buffer.

For consistency, I've removed strbuf_catf, and converted all uses of
it into the new put_fmt - and I've also added an extra vtable method
in the BinarySink API, so that put_fmt can still use strbuf_catf's
more efficient memory management when talking to a strbuf, and fall
back to the simpler strategy when that's not available.

utils/antispoof.c

@@ -18,7 +18,7 @@ void seat_antispoof_msg(InteractionReadySeat iseat, const char *msg)
          * wouldn't be able to mimic it within our line-length
          * constraint.
          */
-        strbuf_catf(sb, "-- %s ", msg);
+        put_fmt(sb, "-- %s ", msg);
         while (sb->len < 78)
             put_byte(sb, '-');
     }