Swap the BC0 and BXN[1:0] = 0 frame markers

It was observed that some very noisy trigger links might never lock. This is due to the comma symbol frame marker having lower precedence than the overflow frame marker. Therefore no guarantee is given whether the frame marker is sent or not.

The proposed solution is to use the K.28.5/0xBC comma symbol as the BC0 frame marker. It is always sent periodically on the link (every 3564 BX) thanks to its highest priority. That is, implement the following table.

Tests with the OTMB and GEM internal tests are scheduled for tomorrow.

Priority	Symbol	Usage	Comment
1	K.28.5 (0xBC)	BC0	Main comma character, sent periodically
2	K.28.1 (0x3C)	ReSync	Auxiliary comma character, forwards TTC ReSync to the trigger link fibers
3	K.30.7 (0xFE)	Overflow
4	K.28.0 (0x1C)	BXN[1:0] = 0
	K.23.7 (0xF7)	BXN[1:0] = 1
	K.27.7 (0xFB)	BXN[1:0] = 2
	K.29.7 (0xFD)	BXN[1:0] = 3

Fixes #44 (closed)