Trim BOM from config file for windows support

[sparrc]

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Default windows text editor (Notepad) adds a BOM to the beginning of the
file. This needs to be trimmed otherwise we will get an "invalid toml"
error.

see influxdata/telegraf#1378
and http://utf8everywhere.org/#faq.boms

[mention-bot]

By analyzing the blame information on this pull request, we identified @gunnaraasen and @e-dard to be potential reviewers

[sparrc]

without this change we get an error when parsing a file with a BOM:

% influxd -config ~/db/ws/influxd.conf.bom

 8888888           .d888 888                   8888888b.  888888b.
   888            d88P"  888                   888  "Y88b 888  "88b
   888            888    888                   888    888 888  .88P
   888   88888b.  888888 888 888  888 888  888 888    888 8888888K.
   888   888 "88b 888    888 888  888  Y8bd8P' 888    888 888  "Y88b
   888   888  888 888    888 888  888   X88K   888    888 888    888
   888   888  888 888    888 Y88b 888 .d8""8b. 888  .d88P 888   d88P
 8888888 888  888 888    888  "Y88888 888  888 8888888P"  8888888P"

[run] 2016/06/23 09:24:27 InfluxDB starting, version unknown, branch unknown, commit unknown
[run] 2016/06/23 09:24:27 Go version go1.6.2, GOMAXPROCS set to 4
[run] 2016/06/23 09:24:27 Using configuration at: /Users/sparrc/db/ws/influxd.conf.bom
run: parse config: Near line 0 (last key parsed ''): Bare keys cannot contain '\ufeff'.

[sparrc]

corresponding PR in telegraf: influxdata/telegraf#1404

[sparrc]

cc @nathanielc in case you want to make this change for Kapacitor as well

[e-dard]

@sparrc I think it would be nice to keep these sort of OS-specific actions in the the OS binaries or runtimes. So I would prefer something in trimBOM like:

if runtime.GOOS != "windows" {
    return fileBytes
}

An alternative approach would to use build flags to compile out the trimBOM function, but that seems a bit overkill in this case. Maybe @jwilder has some thoughts on this?

[sparrc]

That seems reasonable. Although one counterpoint is that the BOM is not necessarily only on Windows. Any editor/OS can add that prefix, but as far as I know Notepad is the only one that actually does.

Also a file that Notepad saves could be transferred to any other machine.

Vim, for example, deals with the BOM on any system. You can validly open a file with a BOM on linux/osx so we may want to do the same.

[e-dard]

Ah right OK. If it's not windows-specific then it's cool as it is. LGTM 👍

[jsternberg]

Update the changelog and I think this is good. I agree with @sparrc that we shouldn't limit this code to only Windows. It's possible for someone to edit a file in Notepad on Windows and then copy it to a Linux machine by using something like Chef or Ansible so I think it's worth always trimming the byte mark since it doesn't really harm the Linux version.

[joelegasse]

This should definitely be bytes.TrimPrefix... bytes.Trim will remove the code points from the beginning and the end. If this is only going to appear at the beginning, we should only check there.

[sparrc]

sure, I'll change it