[wip] telegraf (metrics collecting agent)

Hi,

here's a wip port for https://github.com/influxdata/telegraf/ - the
agent collecting metrics from influxdata, goes well with influxdb.
it's go so we need to vendor dependencies as usual, and the resulting
binary is 100mb, but it seems it's able to collect basic system
statistics with its default config:

$telegraf -test -debug
2018/11/15 18:53:37 I! Using config file: /etc/telegraf/telegraf.conf
> disk,device=sd0a,fstype=ffs,host=c64.proxmox2,mode=rw,path=/ free=831016960i,inodes_free=152330i,inodes_total=155902i,inodes_used=3572i,total=1053571072i,used=169877504i,used_percent=16.972569047999052 1542304418000000000
> diskio,host=c64.proxmox2,name=sd0 io_time=0i,iops_in_progress=0i,read_bytes=164071115264i,read_time=0i,reads=10460843i,weighted_io_time=0i,write_bytes=232144855040i,write_time=0i,writes=21308001i 1542304418000000000
> mem,host=c64.proxmox2 active=151158784i,available=3260960768i,available_percent=78.62288748633465,buffered=614514688i,cached=0i,commit_limit=0i,committed_as=0i,dirty=0i,free=3140579328i,high_free=0i,high_total=0i,huge_page_size=0i,huge_pages_free=0i,huge_pages_total=0i,inactive=120381440i,low_free=0i,low_total=0i,mapped=0i,page_tables=0i,shared=0i,slab=0i,swap_cached=0i,swap_free=0i,swap_total=0i,total=4147597312i,used=886636544i,used_percent=21.377112513665356,vmalloc_chunk=0i,vmalloc_total=0i,vmalloc_used=0i,wired=4096i,write_back=0i,write_back_tmp=0i 1542304418000000000
> processes,host=c64.proxmox2 blocked=0i,idle=62i,running=3i,sleeping=14i,stopped=0i,total=79i,unknown=0i,zombies=0i 1542304418000000000
> swap,host=c64.proxmox2 free=4298502144i,total=4546478080i,used=247975936i,used_percent=5.454242418782321 1542304418000000000

havent tried it much more but there shouldnt be much to do to actually
finish the port, mostly testing in real life is needed..

feedback welcome.

Landry