[XrdCl] Add errNotImplemented error code
[XrdCl] Add PlugInMgr logging topic
[XrdCl] Add utility functions

1) for parsing config files
2) for getting directory entries
[XrdCl] Implement plug-in interfaces and a plug-in manager
[XrdCl] Add plug-in hooks to XrdCl::File
[XrdCl] Add plug-in hooks to XrdCl::FileSystem
[Man] Add info about plug-in related envvars
[XrdClTests] Test the client plug-ins