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/utils/split_into_argv.c

@@ -514,7 +514,7 @@ int main(int argc, char **argv)
             strbuf *cmdline = strbuf_new();
             char *p;
 
-            strbuf_catf(cmdline, "%s -splat ", argv[0]);
+            put_fmt(cmdline, "%s -splat ", argv[0]);
             printf("    {\"");
             size_t args_start = cmdline->len;
             for (p = argv[2]; *p; p++) {