Avoid leakage of file descriptors in LLDB and LLGS

Summary:
Both LLDB and LLGS are leaking file descriptors into the debugged process. This plugs the leak by
closing the unneeded descriptors. In one case I use O_CLOEXEC, which I hope is supported on
relevant platforms. I also added a regression test and plugged a fd leak in dosep.py.

Reviewers: vharron, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D7372

llvm-svn: 228130