Follow-up to 82245eaa: Fix the example program sendrecv.c (handle
CURLE_AGAIN, handle incomplete send). Improve the documentation
for curl_easy_recv() and curl_easy_send().

Reviewed-by: Frank Meier
Assisted-by: Jay Satiro

See https://github.com/curl/curl/pull/1134

It doesn't benefit us much as the connection could get closed at
any time, and also by checking we lose the ability to determine
if the socket was closed by reading zero bytes.

Reported-by: Michael Kaufmann

Closes https://github.com/curl/curl/pull/1134

As the official docs seems really hard to keep track of and link to over
time

Fixed trailing whitespace and numerous formatting glitches

--krb4, --ftp-ssl and --ftp-ssl-reqd no longer need to be documented in the
man page

This is the first time we replace the manually edited curt.1 with the
generated one created by gen.pl and the individual option documentation
pages.

Do not edit this file, edit the individual pages and regenerate this
output.

This file will be generated by the build system soon and then removed
from git.

... to avoid people from trying to edit the pending curl.1 version that
gets generated by gen.pl

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