Fixed an issue where if the operating system python plug-in is changed at...

Fixed an issue where if the operating system python plug-in is changed at runtime, it wouldn't cause the process to reload the new operating system plug-in, now it does.

This is currently controlled by a setting:

(lldb) settings set target.process.python-os-plugin-path <path>

Or clearing it with:

(lldb) settings clear target.process.python-os-plugin-path 

The process will now reload the OperatingSystem plug-in.

This was implemented by:
- adding the ability to set a notify callback for when an option value is changed
- added the ability for the process plug-in to load the operating system plug-in on the fly
- fixed bugs in the Process::GetStatus() so all threads are displayed if their thread IDs are larger than 32 bits
- adding a callback in ProcessProperties to tell when the "python-os-plugin-path" is changed by the user
- fixing a crasher in ProcessMachCore that happens when updating the thread list when the OS plugin is reloaded

llvm-svn: 225831