Implement remote process listing in Linux platform.

Summary:

Test Plan: Connect to a remote implementing the platform protocol (ds2 in this case), run `platform process list` and see processes being displayed.

Reviewers: vharron, tfiala, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D6571

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@223752 91177308-0d34-0410-b5e6-96231b3b80d8