Use threadsafe getpwnam_r
Add support to attach the linked-in (default) Acc library of XRootD

This is done by specifying "default" as library name in the configuration line.
Small change for adding an = to a debug msg
Set the uid and gid in the env for further use by plugins
- avoid SEGV if entity->name is not defined by the security plugin
- map everybody not in the password DB to nobody/nobody