Adds access to the effectively used protocol/scheme to both libcurl and
curl, both in string and numeric (CURLPROTO_*) form.

Note that the string form will be uppercase, as it is just the internal
string.

As these strings are declared internally as const, and all other strings
returned by curl_easy_getinfo() are de-facto const as well, string
handling in getinfo.c got const-ified.

Closes #1137

Detects (char*) and 'char*foo' uses.

Verifies a 'char *name' style, with no space after the asterisk.

Coverity CID 1394666

It builds and runs all test cases. No adaptations for actual HTTPS proxy
support has been made.

vtls/gtls.c: In function ‘Curl_gtls_data_pending’:
vtls/gtls.c:1429:3: error: this ‘if’ clause does not guard... [-Werror=misleading-indentation]
   if(conn->proxy_ssl[connindex].session &&
      ^~
      vtls/gtls.c:1433:5: note: ...this statement, but the latter is misleadingly indented as if it is guarded by the ‘if’
           return res;

* HTTPS proxies:

An HTTPS proxy receives all transactions over an SSL/TLS connection.
Once a secure connection with the proxy is established, the user agent
uses the proxy as usual, including sending CONNECT requests to instruct
the proxy to establish a [usually secure] TCP tunnel with an origin
server. HTTPS proxies protect nearly all aspects of user-proxy
communications as opposed to HTTP proxies that receive all requests
(including CONNECT requests) in vulnerable clear text.

With HTTPS proxies, it is possible to have two concurrent _nested_
SSL/TLS sessions: the "outer" one between the user agent and the proxy
and the "inner" one between the user agent and the origin server
(through the proxy). This change adds supports for such nested sessions
as well.

A secure connection with a proxy requires its own set of the usual SSL
options (their actual descriptions differ and need polishing, see TODO):

  --proxy-cacert FILE        CA certificate to verify peer against
  --proxy-capath DIR         CA directory to verify peer against
  --proxy-cert CERT[:PASSWD] Client certificate file and password
  --proxy-cert-type TYPE     Certificate file type (DER/PEM/ENG)
  --proxy-ciphers LIST       SSL ciphers to use
  --proxy-crlfile FILE       Get a CRL list in PEM format from the file
  --proxy-insecure           Allow connections to proxies with bad certs
  --proxy-key KEY            Private key file name
  --proxy-key-type TYPE      Private key file type (DER/PEM/ENG)
  --proxy-pass PASS          Pass phrase for the private key
  --proxy-ssl-allow-beast    Allow security flaw to improve interop
  --proxy-sslv2              Use SSLv2
  --proxy-sslv3              Use SSLv3
  --proxy-tlsv1              Use TLSv1
  --proxy-tlsuser USER       TLS username
  --proxy-tlspassword STRING TLS password
  --proxy-tlsauthtype STRING TLS authentication type (default SRP)

All --proxy-foo options are independent from their --foo counterparts,
except --proxy-crlfile which defaults to --crlfile and --proxy-capath
which defaults to --capath.

Curl now also supports %{proxy_ssl_verify_result} --write-out variable,
similar to the existing %{ssl_verify_result} variable.

Supported backends: OpenSSL, GnuTLS, and NSS.

* A SOCKS proxy + HTTP/HTTPS proxy combination:

If both --socks* and --proxy options are given, Curl first connects to
the SOCKS proxy and then connects (through SOCKS) to the HTTP or HTTPS
proxy.

TODO: Update documentation for the new APIs and --proxy-* options.
Look for "Added in 7.XXX" marks.

This is done for all functions of the form Curl_read[136][624]_[lb]e.

See CRL-01-006.

- Fix connection reuse for when the proposed new conn 'needle' has a
specified local port but does not have a specified device interface.

Bug: https://curl.haxx.se/mail/lib-2016-11/0137.html
Reported-by: bjt3[at]hotmail.com

Fix warnings about conversions from long to time_t in openssl.c and
schannel.c.

Follow-up to de4de4e3

Visual C++ now complains about implicitly casting time_t (64-bit) to
long (32-bit). Fix this by changing some variables from long to time_t,
or explicitly casting to long where the public interface would be
affected.

Closes #1131

When reusing a connection, make sure the unix domain
socket option matches.

Follow-up to a4d88885. Changing the window size in that commit resulted
in a different HTTP2-Settings upgrade header, causing test 1800 to fail.

- Improve performance by using a huge HTTP/2 window size.

Bug: https://github.com/curl/curl/issues/1102


Reported-by:  <afrind@users.noreply.github.com>
Assisted-by: Tatsuhiro Tsujikawa

and warn on unrecognized lines

... and warn for too long --help lines

- In Curl_http2_switched don't call memcpy when src is NULL.

Curl_http2_switched can be called like:

Curl_http2_switched(conn, NULL, 0);

.. and prior to this change memcpy was then called like:

memcpy(dest, NULL, 0)

.. causing address sanitizer to warn:

http2.c:2057:3: runtime error: null pointer passed as argument 2, which
is declared to never be null

Reviewed-by: Nick Zitzmann

Closes #1105

Fixes test 1139 failures

Follow-up to f82bbe01