Fix init script (elastic#4432)

* Correct the way runuser is called
* Only use `-u $beat_user` if `beat_user` is not `root`. That's because the `-u` command seems to have issues on ubuntu systems.

Fixes elastic#4429.

Use .go-version to specify the Go version for all CI builds (elastic#…

…4303) (elastic#4403)

Having a simple file that requires no parsing to retrieve the Go version
provides us a standard portable way to know what Go version to use for builds.
It's basically the least common denominator for builds accross CI systems
(Jenkins, AppVeyor, Travis) and operating systems.

Also changed AppVeyor to invalidate the cached Go version only when the
.go-version file changes instead of when the .appveyor.yml changes.
(cherry picked from commit 795ceab)

Fix non-deterministic vsphere datastore tests (elastic#4220)

The data used in the test appears to be simulated using `df -k`. So the assertions needed to be less strict.

And on windows, `df -k` doesn’t exist so the values are 0 which caused a panic due to a `panic: runtime error: integer divide by zero`.

Update how-filebeat-works.asciidoc (elastic#4199)

Fix console output (elastic#4045) (elastic#4130)

* fix missing return

(cherry picked from commit 18525be)

Update docs.asciidoc (elastic#4095)

fsstats doesn't work anymore because of a previous merge. 
Would be helpful to change fsstats -> fsstat to avoid confusion.

Update yaml tips (elastic#3996)

Improve handling of different path variants on Windows (elastic#3781) (…

…elastic#3786)

* Improve handling of different path variants on Windows

If forward slashes were used on Windows the glob was not matching on startup which lead to the issue that data was resent. This is solved in 5.3 by using path.Abs for all paths which also includes Clean. To make sure Path and Glob are always clean, Cleanup was added to the MatchFile part. This makes sure in case old data / incorrect data is loaded, the comparison will still work. More tests were added for windows to verify change.

Before this change, option 1 below did not work.

```
"F:/wwwLogs/flights-wsapi/WebServicesWebApi.log"
"F:\\wwwLogs\\flights-wsapi\\WebServicesWebApi.log"
'F:\wwwLogs\flights-wsapi\WebServicesWebApi.log'
```

Based on https://discuss.elastic.co/t/duplicate-events-with-filebeat-on-windows-on-service-restart/78743/10

* Move clean path outside for loop

(cherry picked from commit 4527162)

Minor edits

Update testing environment for 5.2.1 (elastic#3564)