Progress: Resume working on custom payload functions.

Progress: Resume working on custom payload functions.

I'm getting more back into things and this is a small step towards pickup up where I left off.
There are several todos that I will consider and either implement or not.

This performs the work that I mentioned in the previous commit in regards to redesigning the approach in fl_fss_payload_header_write() (now called fl_fss_payload_header_map()).