Avoid using microtime(9) for atime/mtime, we don't need to have it

THAT accurate and microtime(9) is painlessly slow on i386 currently.
This speeds up small transfers much. The gain for large transfers
is less significant, but notable too.
Bottleneck was found by Andreas Persson (Re: kern/14246).

Performance improvement with PIII on 661 Mhz according to hbench (with
PIPE_MINDIRECT=8192):

buffersize     before    after
512            17        49
1024           33        110
2048           52        143
4096           77        163
8192           142       190
64K            577       662
128K           372       392

sys/kern/sys_pipe.c

@@ -1,4 +1,4 @@
-/*	$NetBSD: sys_pipe.c,v 1.16 2001/10/08 07:50:17 mycroft Exp $	*/
+/*	$NetBSD: sys_pipe.c,v 1.17 2001/10/28 20:47:15 jdolecek Exp $	*/
 
 /*
  * Copyright (c) 1996 John S. Dyson
@@ -82,12 +82,18 @@
 #include <sys/syscallargs.h>
 #include <uvm/uvm.h>
 #include <sys/sysctl.h>
+#include <sys/kernel.h>
 #endif /* NetBSD, FreeBSD */
 
 #include <sys/pipe.h>
 
 #ifdef __NetBSD__
-#define vfs_timestamp(tv) 	microtime(tv)
+/*
+ * Avoid microtime(9), it's slow. We don't guard the read from time(9)
+ * with splclock(9) since we don't actually need to be THAT sure the access
+ * is atomic.
+ */
+#define vfs_timestamp(tv)	(*(tv) = time)
 #endif
 
 /*