PoolSvc: make check for ATLAS_POOL_COND_PATH autofs compliant
We have been seeing very strange and irreproducible failures in one of our unit tests in accessing the default pool catalogue. The full discussion can be found in ATR-28801 but the bottom line is that it may be related to the use of autofs for the cvmfs mounts. The change here may fix this:
Use fopen instead of stat to check for the existence of the default pool catalog. The latter may return a false negative in case the file is residing on an autofs as mounting is guaranteed only for "any access beyond a stat".
If the fix is successful in main will chery-pick this into 24.0 as well.