syscall: fix marshaling of stat messages on Plan 9

The order of the Qid fields was reversed. Mea culpa.

R=seed
CC=golang-dev
https://codereview.appspot.com/7231062
cmd/gc: fix escape analysis

If the analysis reached a node twice, then the analysis was cut off.
However, if the second arrival is at a lower depth (closer to escaping)
then it is important to repeat the traversal.

The repeating must be cut off at some point to avoid the occasional
infinite recursion. This CL cuts it off as soon as possible while still
passing all tests.

Fixes issue 4751.

R=ken2
CC=golang-dev, lvd
https://codereview.appspot.com/7303043