Fix FileMutex and other cases

[machour]

I believe we should do a global code review on this repo.

For example, in https://github.com/yiisoft/mutex/blob/master/src/FileMutex.php#L66 isWindows never gets a chance to be anything other than null. Probably a mistake while migrating from init().

Tests didn't pick that up, so we also need to increase coverage.

[samdark]

Ideally that should be done for every package...