http(s): automatically try NTLM authentication first

It is common in corporate setups to have permissions managed via a
domain account. That means that the user does not really have to log in
when accessing a central repository via https://, but that the login
credentials are used to authenticate with that repository.

The common way to do that used to require empty credentials, i.e. hitting
Enter twice when being asked for user name and password, or by using the
very funny notation https://:@server/repository

A recent commit (5275c30 (http: http.emptyauth should allow empty (not
just NULL) usernames, 2016-10-04)) broke that usage, though, all of a
sudden requiring users to set http.emptyAuth = true.

Which brings us to the bigger question why http.emptyAuth defaults to
false, to begin with.

It would be one thing if cURL would not let the user specify credentials
interactively after attempting NTLM authentication (i.e. login
credentials), but that is not the case.

It would be another thing if attempting NTLM authentication was not
usually what users need to do when trying to authenticate via https://.
But that is also not the case.

So let's just go ahead and change the default, and unbreak the NTLM
authentication. As a bonus, this also makes the "you need to hit Enter
twice" (which is hard to explain: why enter empty credentials when you
want to authenticate with your login credentials?) and the ":@" hack
(which is also pretty, pretty hard to explain to users) obsolete.

This fixes #987

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

http.c

@@ -109,7 +109,7 @@ static int curl_save_cookies;
 struct credential http_auth = CREDENTIAL_INIT;
 static int http_proactive_auth;
 static const char *user_agent;
-static int curl_empty_auth;
+static int curl_empty_auth = 1;
 
 #if LIBCURL_VERSION_NUM >= 0x071700
 /* Use CURLOPT_KEYPASSWD as is */