contrib/signet/miner: Automatic timestamp for first block

When mining the first block of a new signet chain, pick a timestamp for
the first block so that after mining 100 blocks the timestamp will be
back to the current time -- this prevents an unnecessary delay before
any miner rewards have matured enough to be spent.  This takes into
account that the delta between blocks may be shorter than 10 minutes due
to attempting to increase the difficulty to match --nbits, but does not
take into account the time spent actually generating the 100 blocks.

contrib/signet/miner

@@ -428,10 +428,13 @@ def do_generate(args):
             action_time = now
             is_mine = True
         elif bestheader["height"] == 0:
-            logging.error("When mining first block in a new signet, must specify --set-block-time")
-            return 1
+            time_delta = next_block_delta(int(bestheader["bits"], 16), bci["bestblockhash"], ultimate_target, args.poisson)
+            time_delta *= 100 # 100 blocks
+            logging.info("Backdating time for first block to %d minutes ago" % (time_delta/60))
+            mine_time = now - time_delta
+            action_time = now
+            is_mine = True
         else:
-
             time_delta = next_block_delta(int(bestheader["bits"], 16), bci["bestblockhash"], ultimate_target, args.poisson)
             mine_time = bestheader["time"] + time_delta