[http] Also allow remote playlists to have https streams (fixes #1251)

Fix it by adding net_is_http_or_https() utility function in misc.h and make
sure it used whenever a http protocol check is made.

src/http.c

@@ -319,7 +319,7 @@ http_stream_setup(char **stream, const char *url)
 	  line = pos;
 	}
 
-      if (strncasecmp(line, "http://", strlen("http://")) == 0)
+      if (net_is_http_or_https(line))
 	{
 	  DPRINTF(E_DBG, L_HTTP, "Found internet playlist stream (line %d): %s\n", n, line);