remove hard dependency on allow_url_fopen = On in ReCaptcha

[nijel]

Currently the code that use ReCaptcha at:
https://github.com/phpmyadmin/phpmyadmin/blob/master/libraries/plugins/auth/AuthenticationCookie.php#L282
depend on allow_url_fopen = On as noted in this bug:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=784628

Remove hard dependency on allow_url_fopen = On in ReCaptcha usage, for example:
change in:
https://github.com/phpmyadmin/phpmyadmin/blob/master/libraries/plugins/auth/AuthenticationCookie.php#L282
the code to something like: (not tested, based on https://github.com/google/recaptcha/blob/master/examples/example-captcha.php#L82)

         if (function_exists('curl_init')) {
                $reCaptcha = new ReCaptcha(
                      $GLOBALS['cfg']['CaptchaLoginPrivateKey'],
                      new ReCaptcha\RequestMethod\CurlPost()
                );
        } else if (ini_get('allow_url_fopen')) {
                $reCaptcha = new ReCaptcha(
                      $GLOBALS['cfg']['CaptchaLoginPrivateKey']
                );
        } else {
                $reCaptcha = new ReCaptcha(
                      $GLOBALS['cfg']['CaptchaLoginPrivateKey'],
                      new ReCaptcha\RequestMethod\SocketPost()
                );
        }

[nijel]

Note for myself:

Once this is released, adjust Debian package:

1. adding hard dependency on php-curl (Depends in: https://sources.debian.net/src/phpmyadmin/4:4.6.4%2Bdfsg1-1/debian/control/?hl=29#L29) & disallow allow_url_fopen in:
   https://sources.debian.net/src/phpmyadmin/4:4.6.4%2Bdfsg1-1/debian/conf/apache.conf/#L20
   https://sources.debian.net/src/phpmyadmin/4:4.6.4%2Bdfsg1-1/debian/conf/apache.conf/#L36
2. don't disallow allow_url_fopen, but add php-curl in "Recommends" in:
   https://sources.debian.net/src/phpmyadmin/4:4.6.4%2Bdfsg1-1/debian/control/?hl=29#L29

[nijel]

Debian package adjusted in https://anonscm.debian.org/git/collab-maint/phpmyadmin.git/commit/?id=458a710459ce35c51d7a871d6f92a537dd64d588