Implement timeouts for exec command runners

[sparrc]

Currently gopsutil has a few places where OS commands are run using the exec library.

Any one of these commands could hang, causing the gopsutil call to hang. While the hang is an OS problem, gopsutil should be able to see it and return an error.

Unfortunately the exec library doesn't have any built-in support for timeouts, but these can be implemented relatively easily using timers (see https://github.com/influxdata/telegraf/blob/master/internal/internal.go#L153).

I likely won't have time to work on this for a while, but I wanted to put the feature request out there, and I will try to find some time for this soon if I can.

[shirou]

#203 is merged. Thank you!