Fix diskusage crash on unsupported platforms + add freebsd

[inve1]

When using flood on an unsupported platform, the app wasn't loading because of this exception:

GET /api/activity-stream?historySnapshot=fiveMin 200 1.189 ms - -
TypeError: diskUsage[process.platform] is not a function
    at DiskUsageService.updateDisks (/usr/home/inve1/flood/server/services/diskUsageService.js:99:39)
    at module.exports (/usr/home/inve1/flood/server/middleware/clientActivityStream.js:46:20)
    at Layer.handle [as handle_request] (/usr/home/inve1/flood/node_modules/express/lib/router/layer.js:95:5)
    at next (/usr/home/inve1/flood/node_modules/express/lib/router/route.js:137:13)
    at module.exports (/usr/home/inve1/flood/server/middleware/eventStream.js:26:3)
    at Layer.handle [as handle_request] (/usr/home/inve1/flood/node_modules/express/lib/router/layer.js:95:5)
    at next (/usr/home/inve1/flood/node_modules/express/lib/router/route.js:137:13)
    at Route.dispatch (/usr/home/inve1/flood/node_modules/express/lib/router/route.js:112:3)
    at Layer.handle [as handle_request] (/usr/home/inve1/flood/node_modules/express/lib/router/layer.js:95:5)
    at /usr/home/inve1/flood/node_modules/express/lib/router/index.js:281:22

Description

I put in a check for supported platforms on that function. Since the freebsd df output is the same as linux I also added it as a supported platform with a small refactor

A note about the maxBuffer change: My df output is currently 31045 bytes long. In my case this is because of hundreds of rar2fs mounts to make rar downloads plex-friendly. Perhaps filtering filesystems like these before parsing them in node could be an option since the disk usage from these is useless. Don't want to handle this in a bugfix pr though.

Motivation and Context

Flood stopped working on freebsd after I pulled master today

How Has This Been Tested?

Ran it on freebsd

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[jfurrow]

LGTM!

[noraj]

LGTM : Looks Good To Me (I had to search lol)