Mentioned in bug #1342

And clarify for curl that --proxy-header now must be used for headers
that are meant for a proxy, and they will not be included if the request
is not for a proxy.

Updated the docs to clarify and the code accordingly, with test 1528 to
verify:

When CURLHEADER_SEPARATE is set and libcurl is asked to send a request
to a proxy but it isn't CONNECT, then _both_ header lists
(CURLOPT_HTTPHEADER and CURLOPT_PROXYHEADER) will be used since the
single request is then made for both the proxy and the server.

... and sorted the list

Modified the logic so that CURLOPT_HEADEROPT now controls if PROXYHEADER
is actually used or not.

Includes docs and new test cases: 1525, 1526 and 1527

Co-written-by: Vijay Panghal

This limitation was removed in commit 0bc4938e

In the initializer for len, there is no prototype for "strlen".
In this statement, there is no prototype for "memcpy".

Also fixed a function name change in the version requirement bump

The information about building with Kerberos4 support was half a year out
of date. We dropped support for that.

Added initial support for --next/-: which will be used to replace the
rather confusing : command line operation what was used for the URL
specific options prototype.

... and mention the --tlsv1.[0-2] options in the --tslv1 entry

Reported-by: Hubert Kario

... and edited language slightly

it is only WHILE added to a multi handle that it can't be used with the
easy interface

This feature specifies the availability of cryptographic
authentication, which can be disabled at compile-time

... since it is never returned since a long while back.

For the avoidance of doubt, show a domain which contains the no-proxy
pattern but not at the top level.

... for debug function

The word CURLOPT_PROXYPORT became detached from its sentence when the
note about the default was added.

Replaced the #define CURL_TIMEOUT_EXPECT_100 in transfer.c with the
CURLOPT_EXPECT_100_TIMEOUT_MS option to make the timeout configurable.