Don't set row to zero, then immediately overwrite it

modified a few comments to be more clear.
changed a loop condition to be more consistent with other loops of the same
type.
removed setting a bit field to zero when we're just going to overwrite it
anyway.
change function return values

reserve (-1) for fatal errors. if we fail to move a block then return 0,
return 1 on sucess.
Rename function from create_block to randomize_block

It doesn't allocate or remove memory, to it's really not creating anything.
change variable width of level

change variable level in struct block_game to 16 bits.
Correct comment about tick delay. It's nanoseconds not milliseconds.