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.
This is aimed at allowing a stride to optionally be
given for the parameter name array... this will allow
use of lws_struct metadata as the parameter name
array.
Also introduce the option to put all allocations in
an lwsac instead of via lws_mallocs.
If you just want a "file" in multipart, don't care about the length or
anything else, then you don't need any params tables and associated
allocations.
Audit all lws_hdr_copy() usages inside lws and make sure we
take care about it failing.
Also since the patch around aggregation of headers by ',',
lws_hdr_copy() needs a little more space in the output buffer,
adjust one place where that caused it to start failing in an
exact-sized buffer.
