IRRHFSU: Add Option to exclude directories or ignore /dsfs and /proc mountpoints in general

[MFleissig]

If IRRHFSU unloads the whole USS structure by specifying / as directory to unload the structure, it would result in tons of RACF access errors because of files below /dsfs for the Dataset Filesystem.

Add an option to exclude selected directories or in general, ignore the /dsfs and /proc mountpoints.