Updated API — Read Desc for Details

New logic in rednet.open
Fixed logic in rednet.openAll
New logic in rednet.close
Fixed logic in rednet.closeAll
Added new function rednet.refresh which checks that all modems that are
registered as open actually are open, this should be done in MOST
rednet functions (updated appropriately)
Added open modem check to MOST rednet functions, should be added to any
new ones that require to use a modem
Added new function advancedReceive should be used in ALL internal
rednet calls as it sends an ACK message in response