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.

windows/storage.c

@@ -318,7 +318,7 @@ void enum_settings_finish(settings_e *e)
 static void hostkey_regname(strbuf *sb, const char *hostname,
                             int port, const char *keytype)
 {
-    strbuf_catf(sb, "%s@%d:", keytype, port);
+    put_fmt(sb, "%s@%d:", keytype, port);
     escape_registry_key(hostname, sb);
 }