Make `true` to be a valid boolean option.
Make `true` to be a valid boolean option.

Currently, `ON` or `1` indicate truth for a boolean option; this also
allows `TRUE`.

cvmfs/cvmfs,DrDaveD/cvmfs,Gangbiao/cvmfs,Gangbiao/cvmfs,Gangbiao/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,Gangbiao/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,Gangbiao/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs