For HTTP/2, we do this validation in libnghttp2.  http-parser does
this partially, when it parses URI, but it does not do anything for
Host header field.  libspdylay does not perform anything.  So do some
additional validation for HTTP/1 and SPDY cases.  integration tests
were also added to make sure they work.

Without this validation, test spuriously succeeds, but in fact it
doesn't.  This might occur if stream is closed with RST_STREAM or
GOAWAY.

We only change req_.upgrade_request once, so just using else is
suffice.

Currently, this does not do anything useful.

Use FieldStore::append_last_{header,trailer}_value instead.

Header field related functions are now gathered into FieldStore class.
This commit only handles request.  Subsequent commit will do the same
thing for response.

This fixes a crash in nghttpx when running on IPv6 only.

Disallow request from server, and response from client respectively.
When the violation is detected, return NGHTTP2_ERR_PROTO from
nghttp2_submit_request, nghttp2_submit_response,
nghttp2_submit_headers.

We also did some refactoring, and now self-dependency detection is
placed where it is only required.