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>

filters/email-gravatar.lua

@@ -16,6 +16,7 @@ end
 
 function filter_close()
 	html("<img src='//www.gravatar.com/avatar/" .. md5 .. "?s=13&amp;d=retro' style='height:10pt;width:10pt' alt='Gravatar' /> " .. buffer)
+	return 0
 end
 
 function filter_write(str)