Improve stability of Linux ProcessMonitor by not using fds for synchronization:
- ProcessMonitor::[Do|Serve]Operation no longer depend on file descriptors!
- removed unused member functions CloseFD and EnableIPC
- add semaphores to signal when an Operation is ready to be processed/complete.

This commit fixes a bug that was identified under stress-testing (i.e. build
LLVM while running tests) that led to LLDB becoming unresponsive because the
read/write operations on file descriptors in ProcessMonitor were not checked.

Other test runner improvement/convenience:
- pickup environment variables LLDB_LINUX_LOG and LLDB_LINUX_LOG_OPTIONS to
  enable (Linux) logging when running the test suite. Example usage:

        $ LLDB_LINUX_LOG="mylog.txt" LLDB_LINUX_LOG_OPTIONS="process thread" python dotest.py




git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@190820 91177308-0d34-0410-b5e6-96231b3b80d8