Add proper support for proxies

There are situations where you want tt-rss to use a proxy (e.g.
because of network restrictions, or privacy concerns).
tt-rss already comes with an undocumented `_CURL_HTTP_PROXY`
variable (see eg https://binfalse.de/2015/05/06/ttrss-with-proxy/),
however that won't have an effect when, for example, php-curl is
not installed, see
https://git.tt-rss.org/git/tt-rss/src/c30f5e18119d1935e8fe6d422053b127e8f4f1b3/include/functions.php#L377
In this case it would use the `file_get_contents` with a stream
context without a proxy definition:
https://git.tt-rss.org/git/tt-rss/src/c30f5e18119d1935e8fe6d422053b127e8f4f1b3/include/functions.php#L487

Here I propose to properly support proxies, and I introduced a
`PROXY` variable, that is respected in both scenarios, with and
without curl installed.

config.php-dist

@@ -196,4 +196,9 @@
 	// Expected config version. Please update this option in config.php
 	// if necessary (after migrating all new options from this file).
 
+	define('PROXY', '');
+	// Connect to RSS feeds through a PROXY, this way tt-rss won't connect to
+	// webservers directly.
+	// Example format: '127.0.0.1:8123' (polipo proxy running on localhost)
+
 	// vim:ft=php

include/functions.php

@@ -402,8 +402,8 @@
 				curl_setopt($ch, CURLOPT_COOKIEJAR, "/dev/null");
 			}
 
-			if (defined('_CURL_HTTP_PROXY')) {
-				curl_setopt($ch, CURLOPT_PROXY, _CURL_HTTP_PROXY);
+			if (defined('PROXY')) {
+				curl_setopt($ch, CURLOPT_PROXY, PROXY);
 			}
 
 			if ($post_query) {
@@ -483,25 +483,24 @@
 
 			// TODO: should this support POST requests or not? idk
 
+			 $context_options = array(
+				  'http' => array(
+						'method' => 'GET',
+					    'ignore_errors' => true,
+					    'timeout' => $timeout ? $timeout : FILE_FETCH_TIMEOUT,
+						'protocol_version'=> 1.1)
+				  );
+
 			if (!$post_query && $last_modified) {
-				 $context = stream_context_create(array(
-					  'http' => array(
-							'method' => 'GET',
-						    'ignore_errors' => true,
-						    'timeout' => $timeout ? $timeout : FILE_FETCH_TIMEOUT,
-							'protocol_version'=> 1.1,
-							'header' => "If-Modified-Since: $last_modified\r\n")
-					  ));
-			} else {
-				 $context = stream_context_create(array(
-					  'http' => array(
-							'method' => 'GET',
-						    'ignore_errors' => true,
-						    'timeout' => $timeout ? $timeout : FILE_FETCH_TIMEOUT,
-							'protocol_version'=> 1.1
-					  )));
+                $context_options['http']['header'] = "If-Modified-Since: $last_modified\r\n";
 			}
 
+			if (defined('PROXY')) {
+				$context_options['http']['proxy'] = PROXY;
+			}
+
+            $context = stream_context_create($context_options);
+
 			$old_error = error_get_last();
 
 			$data = @file_get_contents($url, false, $context);