CURLOPT_SOCKS_PROXY -> CURLOPT_PRE_PROXY

Added the corresponding --preroxy command line option. Sets a SOCKS
proxy to connect to _before_ connecting to a HTTP(S) proxy.

... the newly introduced CURLOPT_SOCKS_PROXY is special and should be
asked for specially. (Needs new code.)

Unified proxy type to a single variable in the config struct.

This was added as part of the SOCKS+HTTPS proxy merge but there's no
need to support this as we prefer to have the protocol specified as a
prefix instead.

... they're already frowned upon in our source code style guide, this
now enforces the rule harder.

In order to make the code style more uniform everywhere

ERR_PACK is an internal detail of OpenSSL. Also, when using it, a
function name must be specified which is overly specific: the test will
break whenever OpenSSL internally change things so that a different
function creates the error.

Closes #1157

Closes https://github.com/curl/curl/pull/1153

... like 'user-agent = ""'

Adjusted test 71 to verify.

Since it now reads responses one byte a time, a loop could be removed
and it is no longer limited to get the whole response within 16K, it is
now instead only limited to 16K maximum header line lengths.

... as they broke with the cleaned up CONNECT handling

... so that it doesn't read data that is actually coming from the
remote. 2xx responses have no body from the proxy, that data is from the
peer.

Fixes #1132

A server MUST NOT send any Transfer-Encoding or Content-Length header
fields in a 2xx (Successful) response to CONNECT. (RFC 7231 section
4.3.6)

Also fixes the three test cases that did this.

Test 1281 added to verify

If a port number in a "connect-to" entry does not match, skip this
entry instead of connecting to port 0.

If a port number in a "connect-to" entry matches, use this entry
and look no further.

Reported-by: Jay Satiro
Assisted-by: Jay Satiro, Daniel Stenberg

Closes #1148

The function only exists since nghttp2 1.12.0.

Bug: https://github.com/curl/curl/commit/a4d8888#commitcomment-19985676
Reported-by: Michael Kaufmann

Closes #1125

Now all options are in the new system.

Moved over to the new format

There's mostly likely no need to allow setting SSLv2/3 version for HTTPS
proxy. Those protocols are insecure by design and deprecated.

Also define it in ILE/RPG binding.

Closes #1142

Add missing tests for CURLINFO_SCHEME, CURLINFO_PROTOCOL, %{scheme},
and %{http_version}

closes #1143

Follow-up to 4f8b1774

Sorted the lists too.

... and include the new ones in the PDF and HTML generation targets