Rename packet constants and file status packet.

Makes it a little bit easier to keep everything straight and consistent.
Implement sending packet containing required file update info.
Refactor send_packet to use length parameter instead of send_size.

send_size will still be incremeneted (and subsequently reduced), but
it makes more sense to be able to specify the length of the packet
at the time of sending in order to get a little more control.
Implement send_header function to remove duplicate code.

Relatively common for 4-byte packet headers to be send as messages,
so it makes sense to write a function to do this.