Previously, we just ignored "Connection" header field.  But HTTP/2
specification actually prohibits few more header fields.  This commit
ignores all of them so that we don't send these bad header fields.

Bug: https://curl.haxx.se/mail/archive-2016-10/0033.html
Reported-by: Ricki Hirner

Closes https://github.com/curl/curl/pull/1092

Fixes #1107

Reported-by: Adam Piggott

Mirrors the autotools behavior introduced with curl-7_50_3-83-ga34c7ce.

Fixes #1089

- Call Curl_initinfo on init and duphandle.

Prior to this change the statistical and informational variables were
simply zeroed by calloc on easy init and duphandle. While zero is the
correct default value for almost all info variables, there is one where
it isn't (filetime initializes to -1).

Bug: https://github.com/curl/curl/issues/1103
Reported-by: Neal Poole

Now showing microsecond resolution.

Closes #1106

Regression added in 62a8095e

Reported-by: Tony Kelman

Discussed in #1087

Reported-by: bemoody

Fixes #1104

and change URL to use https

Reported-by: Bruce Stephens

Fixes #1098

Follow-up to 502acba2

Coverity CID 1374359 pointed out the unused result value.

Coverity CID 1374358

.. because including those headers helps Visual Studio's Intellisense.

..because they're no longer needed in the tool build. strcase is still
built by the libcurl project and exports curl_str(n)equal which is used
by the curl tool.

Bug: https://github.com/curl/curl/commit/9363f1a#all_commit_comments

... and now it avoids using the libcurl toupper() function

follow-up to 46133aa5

...to use the public function curl_strnequal(). This isn't ideal because
it adds extra overhead to any internal calls to checkprefix.

follow-up to 95bd2b3e

Suggested-by: Dan Jacobson
Issue: https://github.com/curl/curl/issues/1097

... since the curlx_* code no longer provides one and we don't link
libcurl to these test servers.

As they are after all part of the public API. Saves space and reduces
complexity. Remove the strcase defines from the curlx_ family.

Suggested-by: Dan Fandrich
Idea: https://curl.haxx.se/mail/lib-2016-10/0136.html

follow-up to 811a693b

This should fix the "warning: 'curl_strequal' redeclared without
dllimport attribute: previous dllimport ignored" message and subsequent
link error on Windows because of the missing CURL_EXTERN on the
prototype.

Follow-up to 811a693b

This is a followup to commit 811a693b

Fix compile break from 811a693b

The internal version strcasecompare isn't available outside libcurl

follow-up to 4f8d0b6f. Since the darwinssl code breaks
otherwise. If you build without darwinssl 10.5 works fine.

as that is still part of the API, fix from 8fe4bd08

Fix bug from 811a693b

Mistake brought by 9c91ec77