Fix racy detection of ally rejecting flashes

Fix racy detection of ally rejecting flashes

sendmessage() checked NF_FLASH on two places: when deciding whether to
send the message, and later when telling the player why it didn't send
a flash.  This can race with the toggle command as follows: if a flash
could not be sent because the recipient's NF_FLASH was off, and the
recipient toggled it on before the flag was checked again, the flash
command claimed the sender wasn't logged on.