Enable debugging thread names in Thread_POSIX

This adds a feature similar to the POCO_WIN32_DEBUGGER_THREAD_NAMES for POSIX threads.

There are a few things the user has to do to enable this:
1) Build POCO with POCO_POSIX_DEBUGGER_THREAD_NAMES
2) Set the environment variable POCO_ENABLE_DEBUGGER (see the Debugging::isAvailable method)
3) Use the debug build