Changes included: 8 changes.

Change bb8434f65da4915e43114611ac1af1dc4e97c57e by lukasz janyst <[log in to unmask]> (3 files): [XrdClTests] Test the client plug-ins

Change ed32ec3ce330adb09a4aad7127df4c6843d8ee01 by lukasz janyst <[log in to unmask]> (1 file): [Man] Add info about plug-in related envvars

Change f406d19c48550c89ed540335e48b1277c10f974c by lukasz janyst <[log in to unmask]> (2 files): [XrdCl] Add plug-in hooks to XrdCl::FileSystem

Change c1b09bb2d088514f5179b48543c4c861a4d117e5 by lukasz janyst <[log in to unmask]> (2 files): [XrdCl] Add plug-in hooks to XrdCl::File

Change 6a2be6f409127c6b29d584005f7e70bc0848e097 by lukasz janyst <[log in to unmask]> (7 files): [XrdCl] Implement plug-in interfaces and a plug-in manager

Change 8d9592359f1d48c4d1502cd1a735c6f14fdd75cc by lukasz janyst <[log in to unmask]> (2 files): [XrdCl] Add utility functions

1) for parsing config files
2) for getting directory entries

Change 0390863d54b22520103649bc9ead25262bbc6d7c by lukasz janyst <[log in to unmask]> (2 files): [XrdCl] Add PlugInMgr logging topic

Change c050f304feb43749d2c0cb5195c6d07ceab21d02 by lukasz janyst <[log in to unmask]> (2 files): [XrdCl] Add errNotImplemented error code