Fix a hang on OSX while executing -exec-run.
Now we wait for input to become available before blocking in fgets.
More details on problem can be found in 
http://lists.cs.uiuc.edu/pipermail/lldb-commits/Week-of-Mon-20141201/014290.html

Patch from dawn@burble.org.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@223222 91177308-0d34-0410-b5e6-96231b3b80d8