Synchronize to xrootd vendors branch:
    - Tiny fix in XrdSecProtocolkrb5.cc (remove a redundant unlock)
    - Improvements in 'xrdgsiproxy':
      - Add support for -exists (-e) switch for the 'info' mode (as in 'grid-proxy-info') to test
        existence and validity of a proxy.
      - Add also support for a clock skew in testing validity (default 30 secs; can be changed with
        the switch -clockskew).


git-svn-id: http://root.cern.ch/svn/root/trunk@32800 27541ba8-7e3a-0410-8455-c3a389f83636