Correctly handle return of getgrgid_r, check for NULL in gEnt

fixes xrootd/xrootd#51
Revert "[XrdCl] Use the built-in poller ::Pause method when stopping and restarting the poller"

This reverts commit fd991cf34a65e8be66b8763c47fd636d3dd84273.
Revert "Extend Pause/Resume to all platforms and remove CPU burner when paused."

This reverts commit c52bc62c6c08d8acd43a3e4fe3d2900c8f124bb1.
Revert "Correct conflict between comments and the code."

This reverts commit d14382b4ae5f2a4ac081afbb18c625006ed39312.
Revert "Initialize implicitly referenced fields to keep Valgrind happy."

This reverts commit ce3315bc2a2247e68af5c795d057108b4d4d91f9.
Revert "Provide a poller pause/resume method to ease forking without spurious errors."

This reverts commit 1f47a789db1d84ad58e69efd2679ec92ce4c8c47.
[XrdCl] Fix coverity issues
Scale XrdSysCondVar::Wait(int sec) to a millisecond wait to avoid
clock tick error.
Update notes on Scale XrdSysCondVar::Wait(int sec) patch.
Avoid SEGV and double free introduced by patch 30ad01b9 (set host).
Update notes on segv/double free patch.
Set reasonable linux thread limit and warn it we cannot do so.
Update notes setting thread limit.
Allow SendData() to send less than the amount of bytes requested.
Update notes on SendData() patch.
Change if tests from macos to APPLE for consistency (Posix will follow).
Add method to check if we are working only in IPV4 mode (needed for proxy).
Redo SfsXio interface to avoid using void pointers as handles.
Add a generic control method to the file/dir and system classes.
Update notes on new OSS methods.