<rdar://problem/12586010>

Python OS plug-ins now fetch thread registers lazily.

Also changed SBCommandInterpreter::HandleCommand() to not take the API lock. The logic here is that from the command line you can execute a command that might result in another thread (like the private process thread) to execute python or run any code that can re-enter the public API. When this happens, a deadlock immediately occurs for things like "process launch" and "process attach".