Fix building the lock on windows
move constants out for windows
Add notification in docs re backup command
apologize profusely about locking on windows
Merge pull request #1961 from barakmich/flock

Fix building the lock on windows
pkg/fileutil: sort files in tests

Readdirnames makes no guarantees about ordering, so sort before testing
the listing. This test was failing consistently on Linux for me.

Also, simplify sort.Strings invocation in wal.