inert_drv: use the mode to indicate activity

Remove the monitored field from the state. Consider a file descriptor to
be unmonitored if both the read and write flags have been cleared.
inert_drv: do not increase fd ulimit on startup

inert uses the fd polling in erts. erts isn't aware the number of file
descriptors has been increased by the driver. Adding fd's above
rlim_cur to driver_select fails.