Add "trusted" capability to OMERO.fs

[jmoore]

Currently to permit scripting it is necessary for an admin to upload a script as an OriginalFile. Trusted files need to be marked so that they are executable. For example,

   ./omero.fs --add /usr/local/path/matlab --trusted

[jmoore]

As discussed during a May 22 call, the most effective solution to this is to use user-mapping so that OS user X's maps to OMERO user X, and unknown OS user unknown to OMERO will not have their files made visible. The OmeroFs process would have to run as root.

For systems where such a mapping doesn't exist, would it make sense to enable "user" directories:

   ./omero.fs --add /var/omero/users/X --mapto X

and any file that gets copied there belongs to user X. The permissions on the directory are then completely up to the sysadmin. OmeroFs will in most cases still need to be run as root.