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.

unix/storage.c

@@ -173,7 +173,7 @@ static char *make_filename(int index, const char *subname)
     if (index == INDEX_SESSION) {
         strbuf *sb = strbuf_new();
         tmp = make_filename(INDEX_SESSIONDIR, NULL);
-        strbuf_catf(sb, "%s/", tmp);
+        put_fmt(sb, "%s/", tmp);
         sfree(tmp);
         make_session_filename(subname, sb);
         return strbuf_to_str(sb);