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devsdp: fix randomization of dial and acceptid

We where allocating the dialid and acceptid using:

rand()<<16 + rand()

this gives a biased values as rand() retuns a 15-bit
number. Instead, use two calls to nrand() to get
the full 32-bit unsigned range.

--- a/sys/src/9/port/devsdp.c

+++ b/sys/src/9/port/devsdp.c

@@ -964,13 +964,13 @@

 		panic("setstate: bad state: %d", state);

 	case CDial:

 		assert(c->state == CInit);

-		c->dialid = (rand()<<16) + rand();

+		c->dialid = (nrand(1<<16)<<16)|nrand(1<<16);

 		convretryinit(c);

 		convoconnect(c, ConOpenRequest, c->dialid, 0);

 		break;

 	case CAccept:

 		assert(c->state == CInit);

-		c->acceptid = (rand()<<16) + rand();

+		c->acceptid = (nrand(1<<16)<<16)|nrand(1<<16);

 		convretryinit(c);

 		convoconnect(c, ConOpenAck, c->dialid, c->acceptid);

 		break;