inert: store state in port driver

Remove the gen_server and store the state in the port driver. The inert
module is now just a set of functions.
inert: add controlling_process/2
test: increase polling

Generate 1024 bytes of random data and read the data byte by byte to
stress the polling mechanism.
inert_drv: return {error, closed}

Instead of crashing the process with a badarg if the port has been
stopped, imitate gen_tcp and return {error, closed}.