Move detach to FreeBSD- and Linux-specific classes.

On Linux there is no separate notion of a process (vs. a thread) for
ptrace(); each thread needs to be individually detached.  On FreeBSD
we have a separate process context, and we detach just it.

Review: http://llvm-reviews.chandlerc.com/D1418


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@189666 91177308-0d34-0410-b5e6-96231b3b80d8