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.

Some platform (e.g., freebsd) does not define _GLIBCXX_USE_C99
and it hides std::to_string functions.

Previously, mkapiref.py required python2.6, as it used format strings,
but continued to use print as a keyword, not a function. But, since 2.6
is implicitly made a requirement, we can also count on print being
available as a function.

This change adds python2.6 as a soft requirement during ./configure,
and converts all print keywords to statements. There's no need to do
anything about the actual building of the doc since sphinx-build can
run under python2 and python3.

The net result is that it no longer matters whether 'python' points to
python2 or python3 (see PEP394), because both will be able to run
mkapiref.py successfully.

This makes the buildsystem forward compatible with automake>=1.14 since
nghttp uses a recursive build. It avoids a large and non-scary warning
on running autoreconf, summarized as:

src/Makefile.am:57: warning: source file 'http-parser/http_parser.c' is in a subdirectory,
src/Makefile.am:57: but option 'subdir-objects' is disabled

Just to make it easier to find the info.