Add support for FreeBSD.

[knz]

This implementation requires to mount the Linux compatibility /proc
filesystem in /compat/linux/proc. Check the FreeBSD handbook for
details.

[ruflin]

@knz Thanks for the contribution. Could you use gofmt -w ... to make sure your code adheres to the golang standard? Currently travis complains.

[knz]

@ruflin thanks for the hint. Also now there is significant shared/redundant code between sigar_linux.go and sigar_freebsd.go. Do you see a way to reduce this redundancy?

[codecov-io]

Current coverage is 73.06%

Merging #33 into master will increase coverage by 1.47%

@@             master        #33   diff @@
==========================================
  Files             9         10     +1   
  Lines          1063        787   -276   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits            761        575   -186   
+ Misses          246        172    -74   
+ Partials         56         40    -16   

Powered by Codecov. Last updated by f42b670...a250079

[tamird]

@knz you might be able to move some things into gosigar_unix, or else create a new file that doesn't end in _GOOS and use build tags to control when it is built.

[knz]

@tamird good idea! I just did this and it works well. Thanks!

[tamird]

looks like there's some new interaction with darwin that is failing in travis.

[knz]

Ping, any news on this one?

[monicasarbu]

@knz thank you for your contribution. I am merging your PR.