Fix descriptor leak in multi-target debugging

Summary:
When debugging two targets concurrently, the pseude terminal master fd from the first one would
leak into the second. This fixes the problem by setting O_CLOEXEC on the master fd. Test
included.

Reviewers: clayborg, vharron

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D7466

llvm-svn: 228570