[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
[XrdClTests] Add missing test plug-in code
Fix errors relating to normal poll() usage (only MacOS affected).
Additional MacOS fixes.
Make sure to initialize rdType as this may cause a SEGV via the bridge.
Make sure to properly set the IPV4 and Private Net flags in bridged protocols.
Avoid Linux semaphore bug that can produce a SEGV.
Update notes on semaphore patch.
[XrdCl] Add possibility to disable client plug-ins for given URL
[RPM] Add example client plugin config
[XrdCl] Make client plug-ins strictly version controled and rename the plugin symbol to XrdClGetPlugIn
[RPM] Use logrotate to handle xrootd logs
Eliminate at the root any hope of crafting malicious headers
Workaround historical bug of XrdOucString (squelches Valgrind warning)