lib: add nghttp2_rcbuf_is_static()

Add a `nghttp2_rcbuf_is_static()` method to tell whether a rcbuf
is statically allocated.

This can be useful for language bindings that wish to avoid
creating duplicate strings for these buffers; concretely, I am
planning to use this in the Node HTTP/2 module that is being
introduced.

Fix bug that forwarded for is not affected by proxy protocol

Fix compile error with mruby 1.3.0

fix: typo

Came up in downstream code review by @lucaslago https://github.com/nodejs/node/pull/14239#discussion_r127539852

README.rst: fix typo

fix up grammar in submit_trailer docs

nghttpx: Verify OCSP response using trusted CA certificates

nghttpx: Set default minimum TLS version to TLSv1.2

Previously, the default minimum TLS version was TLSv1.1, but the
default cipher list didn't include any compatible ciphers with it.
This made handshake fail if TLSv1.1 was negotiated because there was
no shared ciphers.  To make the default settings consistent, the
default minimum TLS version is now TLSv1.2.

fix cleaning in out-of-tree builds

The altered previously failed if the rst sources hadn't been copied over.

It looks like we can use nocopy version here.  We use nocopy version
in frontend in day 1.

spelling mistake in arguments to build nghttp apps