Threaded inferior support for FreeBSD

Modelled in part on GDBRemoteCommunicationClient.

Review: http://llvm-reviews.chandlerc.com/D2267


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@196787 91177308-0d34-0410-b5e6-96231b3b80d8