lua/uloop: fd_add() better args checking

lua/uloop: fd_add() better args checking

Actually check for flags being valid, instead of simply ignoring the
call if flags was zero.

Use standard lua checks for the function argument, so you can get a
normal "argument #2 was invalid, expected function, got xxx" instead of
the vague, "invalid arg list"

Signed-off-by: Karl Palsson <karlp@etactica.com>