libwebsockets

client: multipart mime generation helpers

lws has been able to generate client multipart mime as shown
in minimal-http-client-post, but it requires a lot of user
boilerplate to handle the boundary, related transaction header,
and multipart headers.

This patch adds a client creation flag to indicate it will
carry multipart mime, which autocreates the boundary string
and applies the transaction header with it, and an api to
form the boundary headers between the different mime parts
and the terminating boundary.