Change pipes to preserve message boundaries.

src/lib9/pipe.c

@@ -3,9 +3,8 @@
 #include <libc.h>
 #include <sys/socket.h>
 
-/* BUG: would like to preserve delimiters on systems that can */
 int
 p9pipe(int fd[2])
 {
-	return socketpair(AF_UNIX, SOCK_STREAM, 0, fd);
+	return socketpair(AF_UNIX, SOCK_DGRAM, 0, fd);
 }