cmount: disable tests on windows + race detector

These either hang or produce incorrect results for reasons I haven't
worked out yet.

cmd/cmount/mount_test.go

@@ -1,6 +1,10 @@
 // +build cmount
 // +build cgo
 // +build linux darwin freebsd windows
+// +build !race !windows
+
+// FIXME this doesn't work with the race detector under Windows either
+// hanging or producing lots of differences.
 
 package cmount