Server line text-encoding-related fixes.

- Handle server lines that contain sequences which are invalid in the server encoding. Previously, these would cause the whole line to be interpreted in ISO-8859-1, but now they're simply replaced with an appropriate replacement character.

- Removed prefs.utf8_locale.

- Change default server encoding from system locale to UTF-8.

- Always populate server->encoding with a non-null value - UTF-8.

Fixes #1198

src/common/hexchat.c

@@ -757,7 +757,6 @@ static void
 xchat_init (void)
 {
 	char buf[3068];
-	const char *cs = NULL;
 
 #ifdef WIN32
 	WSADATA wsadata;
@@ -795,9 +794,6 @@ xchat_init (void)
 #endif
 #endif
 
-	if (g_get_charset (&cs))
-		prefs.utf8_locale = TRUE;
-
 	load_text_events ();
 	sound_load ();
 	notify_load ();