Allow use of read-only account as well as read-write with file share distribution points

[HarryStrandJamf]

Need to be able to specify whether to use the read-write or read-only account with a file share distribution point. When the read-only account is used, then it should only allow the distribution point to be used as a source and not a destination. This will allow for increased security when a file share distribution point should only be used as a source.

This is related to the comment from @smurf0815 on this issue: #10