spa: determine final boundary
spa: determine final boundary

By itself the HTTP layer can track transaction sizes only with
content-length, including on POST.

However it's also possible for whatever logically interprets
the payload to determine its length, for example with multipart,
the multipart headers can do that job.

This allows the spa stuff to drive the content length tracking,
so lws can interpret multipart POST client payload without needing
an overall content-length.
sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets