dbuck: Make it portable
dbuck: Make it portable

Summary:
Fixes: #1044.

Decompose non portable one liner for killing older debugger instances
and make it work on Linux, BSD and Mac OS X.
Closes https://github.com/facebook/buck/pull/1051

Test Plan:
Start one instance of dbuck
Start another instance
Confirm that the first instance was killed

Reviewed By: k21

fbshipit-source-id: fcb91f6

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