plan9 support

bmo% go test -run 'TestLock$' -v
=== RUN TestLock
--- PASS: TestLock (0.22s)
	lock_test.go:94: Locking in crashing child...
	lock_test.go:87: Child output: "" (err <nil>)
	lock_test.go:99: Locking+unlocking in child...
	lock_test.go:87: Child output: "PASS\n" (err <nil>)
	lock_test.go:104: Locking in parent...
	lock_test.go:110: Again in parent...
	lock_test.go:116: Locking in child...
	lock_test.go:87: Child output: "2014/07/27 21:48:05 Lock failed: Lock Create of /tmp/748837139/foo.lock (abs: /tmp/748837139/foo.lock) failed: open /tmp/748837139/foo.lock: '/tmp/748837139/foo.lock' open/create -- file is locked\n" (err exit status: 'lock.test 152796: 1')
	lock_test.go:121: Unlocking lock in parent
PASS
ok  	github.com/camlistore/lock	0.242s