Correctly handle return of getgrgid_r, check for NULL in gEnt

fixes xrootd/xrootd#51
Merge branch 'fix_groupname' of https://github.com/mgmarino/xrootd
[CMake] XrdSys/XrdSysIOEvents.hh is a private header
[RPM] Add possibility to preload custom allocators to the sysconfig file
Add missing headers to the public headers list.
Merge branch 'master' of github.com:xrootd/xrootd
Allow query of current role and dynamic cms state via kXR_query.
Update notes on kXR_query enhancement.
[XrdSys] Safe Linux Semaphore, take one
[XrdCl] Make the new client use the private semaphore implementation
Fix potential buffer overrun introduced by query config patch.
[XrdSys] Refactor LinuxSemaphore for more clarity
Merge branch 'master' of github.com:xrootd/xrootd