Don't fail if key files are group readable

   This allows server side certificates to be shared among members of a dedicated group.
   Remove any usage of XrdSysPriv

   To remove dependency from setresuid and the related issues in multi-threaded applications.
   Remove any usage of XrdSysPriv

   To remove dependency from setresuid and the related issues in multi-threaded applications.
   Also add functional access check at the keytab during the Init phase on the server side.
   For the time being a check failure implies plug-in load failure. This may be reviewed in
   the future.