libwebsockets

introduce lws_hdr_copy_fragment

This adds a public API variant of the header copy api that lets you
choose which fragment you want copied.

Normally you want the existing one that aggregates the fragments.

But it can be useful to get each part in turn (that corresponds to
the content provided by each duplicated header normally).

Signed-off-by: Andy Green <andy.green@linaro.org>