Add flag to use last access time (where supported) to determine age for file removal

[atc0005]

Either instead of last modification time or in an either/or scenario:

• if X days since last modified
• if Y days since last access

The scenario is pruning files from a temporary directory unpacked by a scanning tool (e.g., data discovery, classification and protection tool). In that scenario, the file modification time is less useful as the files are unmodified from their packed format and could be from years ago, but the access time would presumably vary based on the last time the file was read/scanned.

I've not prototyped this assumption, so it might be that the original access time is preserved making this flag/feature less useful.

I vaguely recall that filesystems can be tuned to not record last access times (noatime ?), which would also make this feature less useful.

For those reasons, it's probably best to add a second flag for using mod time and make it the default behavior if neither flag is specified.