Add generic function to fill buffer with data.  Use this function in the get_key function.
Add function to drain buffers.

I'm realizing that both the fill and drain buffer functions should
be using fread/fwrite functions instead.
Have drain_buf detect complete write to eliminate unnecessary 0 write.
Add loop to pull data from stdin and dump it back to stdout till EOF to simulate encryption with a 'NULL' algorithm.
Remove debug code that dumps the encryption key to stdout.

This makes the buffer hexdump function unnecessary.