Make the debug output less likely to clash with output from the callbacks

[jonathangjertsen]

Sometimes it's useful to turn NMBS_DEBUG on, but the output is a bit of a mess if the read/write callbacks also log to the same channel. Moving the DEBUG statements in recv_msg_footer and send_msg_footer to the start of those functions makes the output a bit cleaner, although it's not perfect.

Example output before this change:

NMBS req -> fc 16   a 12326 q 2     b 4     regs 4660 43981 transmitted[13] = { f7, 10, 30, 26, 00, 02, 04, 12, 34, ab, cd, c2, 04, }

received[1] = { f7, }
received[1] = { 10, }
NMBS res <- fc 16   received[4] = { 30, 26, 00, 02, }
a 12326     q 2received[2] = { bb, 95, }

NMBS req -> fc 3    a 12326 q 2 transmitted[8] = { f7, 03, 30, 26, 00, 02, 3e, 56, }

received[1] = { f7, }
received[1] = { 03, }
NMBS res <- fc 3    received[1] = { 04, }
b 4 received[4] = { 12, 34, ab, cd, }
regs 466043981received[2] = { 96, 2f, }

after:

NMBS req -> fc 16   a 12326 q 2     b 4     regs 4660 43981
transmitted[13] = { f7, 10, 30, 26, 00, 02, 04, 12, 34, ab, cd, c2, 04, }
received[1] = { f7, }
received[1] = { 10, }
NMBS res <- fc 16   received[4] = { 30, 26, 00, 02, }
a 12326     q 2
received[2] = { bb, 95, }
NMBS req -> fc 3    a 12326 q 2
transmitted[8] = { f7, 03, 30, 26, 00, 02, 3e, 56, }
received[1] = { f7, }
received[1] = { 03, }
NMBS res <- fc 3    received[1] = { 04, }
b 4 received[4] = { 12, 34, ab, cd, }
regs 466043981
received[2] = { 96, 2f, }