And utilize it in nghttp to limit initial max concurrent streams.

This function is similar to nghttp2_submit_request and the
difference is it takes an array of nghttp2_nv as name/value pairs.
It is useful if name/value pairs is not NULL-terminated in the
application code.

The reordering iv in the comment is no longer applied to the current
code base, therefore it is removed and iv is made to const pointer.

To make it less likely that a user gets a buffer overflow if a
too small buffer is used.