filter: allow returning exit code from filter

Filters can now indicate a status back to cgit by means of the exit code
for exec, or the return value from close for Lua.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

cgitrc.5.txt

@@ -590,7 +590,8 @@ specification with the relevant string; available values are:
 		This is called whenever cgit writes data to the webpage.
 	'filter_close()'::
 		This is called when the current filtering operation is
-		completed.
+		completed. It must return an integer value. Usually 0
+		indicates success.
 	
 	Additionally, cgit exposes to the Lua the following built-in functions: