Store computed reception time in the message struct so we don't rely on

the message being emitted immediately.

Fix computation of reception time so it's more sensible (the block timestamp
is some time after reception of the _end_ of the block, not the start) - this
means that message-emission times are always later than message-reception
times in SBS output, which is a bit more sensible.

Use clock_gettime in preference to ftime.

demod_2400.c

@@ -425,6 +425,12 @@ void demodulate2400(uint16_t *m, uint32_t mlen) {
 
         // Set initial mm structure details
         mm.timestampMsg = Modes.timestampBlk + (j*5) + bestphase;
+
+        // compute message receive time as block-start-time + difference in the 12MHz clock
+        mm.sysTimestampMsg = Modes.stSystemTimeBlk; // end of block time
+        mm.sysTimestampMsg.tv_nsec -= receiveclock_ns_elapsed(mm.timestampMsg, Modes.timestampBlk + MODES_ASYNC_BUF_SAMPLES * 5); // time until end of block
+        normalize_timespec(&mm.sysTimestampMsg);
+
         mm.score = bestscore;
         mm.bFlags = mm.correctedbits   = 0;