Don't update the channel offset in pread.

The pread syscall is not supposed to update the channel offset,
unless when reading directories.

Reference: /n/sources/patch/pread-offset
Date: Tue Apr 19 18:27:56 GMT 2016
Signed-off-by: djc@9grid.fr

--- /sys/src/9/port/sysfile.c	Tue Apr 19 18:27:51 2016
+++ /sys/src/9/port/sysfile.c	Tue Apr 19 18:27:49 2016
@@ -680,10 +680,12 @@
 	}else
 		nnn = nn = devtab[c->type]->read(c, p, n, off);
 
-	lock(c);
-	c->devoffset += nn;
-	c->offset += nnn;
-	unlock(c);
+	if(c->qid.type & QTDIR || offp == nil){
+		lock(c);
+		c->devoffset += nn;
+		c->offset += nnn;
+		unlock(c);
+	}
 
 	poperror();
 	cclose(c);