Improve Command() handling and signal handling for launched processes.

[sean-]

Use Go's context package for command timeouts.

[shirou]

Thank you! It is exactly what I want to do. But context is a golang 1.8 feature so break some compatibility, could you wait merge until next patch release? gopsutil has monthly release policy, this means April 30.

[sean-]

... yes, I can wait until Monday to have this merged. :) LMK if you have any other concerns. Because this compatibility would be broken along API boundaries due to the removed symbols (I couldn't find any references to these exported, but unused symbols), it didn't seem problematic to merge. The dependency on context is a new dependency, but seems improbable that someone would suddenly need to compile current gopsutils without go1.8 available to them (static compilation of binaries ftw).

[shirou]

Thank you. WaitTimeout and CombinedOutputTimeout are inside internal, so other applications can not use these. And I will write README to note new golang 1.8 dependency after merge.