Due to the layout of the singletest function there are situations where
it returns before it clears the environment variables that were
especially set for the single specific test case. That could lead to
subsequent tests getting executed with environment variables sticking
around from a previous test which could lead to badness.

This change makes sure to clear all custom variables that may be laying
around from a previous round, before running a test case.

Reported by: Kamil Dudka
Bug: http://curl.haxx.se/mail/lib-2010-08/0141.html

warning: conversion to 'long int' from 'time_t' may alter its value

... on win64 when time_t is 64bit and long is 32bit.

The callbacks are called when curl_multi_socket_action() is called, not
when handles are added. This is now mentioned in the "TYPICAL USAGE"
section.

When the progress callback is called during the TCP connection, an error
return would accidentally not abort the operation as intended but would
instead be counted as a failure to connect to that particular IP and
libcurl would just continue to try the next. I made singleipconnect()
and trynextip() return CURLcode properly.

Added bonus: it corrected the error code for bad --interface usages,
like tested in test 1084 and test 1085.

Reported by: Adam Light
Bug: http://curl.haxx.se/mail/lib-2010-08/0105.html

Final fix (hopefully!) for dll wlink loader;
prefer faster internal rm if available.

Added the -br switch to dynamic builds which fixes the issue I saw
with curl's --version output. Added debug info and symfile for debug
builds to linker opts. Added DLL loader for wlink back, but this time
dependend on wlink version.
Patch posted to the list by malak.jiri AT gmail.com.

The var %MAKEFLAGS is only set in 3 cases: if set as environment
var or as macro definition from commandline, and either with the
-u or -ms switch. Since all these cases are unlikely for the average
user it should be safe to only test if %MAKEFLAGS is defined; this
has the benefit that now all other switches can be used again in
addition to the -u which was formerly not possible.

Curl_llist_init is never used outside of llist.c and thus it should be
static. I also removed the protos for Curl_llist_insert_prev and
Curl_llist_remove_next which are functions we removed from llist.c ages
ago.

Test 563 is enabled now and verifies that the combo FTP type=A URL,
CURLOPT_PORT set and proxy work fine. As a bonus I managed to remove the
somewhat odd FTP check in parse_remote_port() and instead converted it
to a better and more generic 'slash_removed' struct field. Checking the
->protocol field isn't right since when an FTP:// URL is sent over a
HTTP proxy, the protocol is HTTP but the URL was handled by the FTP code
and thus slash_removed is set TRUE for this case.

The struct used for storing the message for a completed transfer is now
no longer allocated separatly but is kept within the main struct kept
for each easy handle so that we avoid one malloc (and the subsequent
free).

In some places where the name 'stream' has been used for naming a
function argument that is in fact settable with a setopt() option we now
call that argument 'userdata' to make it more obvious that it is in fact
possible to set by the application.

Suggested by: Jeff Pohlmeyer

When libcurl internally decided to wait for a 100-continue header, there
was no call to the timeout function so there was no timeout callback
called when the multi_socket API was used and thus applications became
either completely wrong or at least ineffecient depending on how they
handled the situation. We now set a timeout to get triggered.

Reported by: Ben Darnell
Bug: http://curl.haxx.se/bug/view.cgi?id=3039744

For now removed the .autodepend directive until I've figured out
which of my changes broke it again.