This enables some systemd sandboxing
options for the `tsm-backup.service`.
Those settings have been determined by expermentation.
This commit tries hard to protect the filesystem from
write access, but not to hide anything from read access,
so users can backup all files they choose to backup.
An exception are API filesystems (`/dev`, `/proc`, `/sys`):
As their "files" are not stored on persistent storage,
they are sandboxed away as much as possible.
Note that the service still has to run with root
privileges to reach files with limited access permissions.
The obvious alternative to use a dedicated user account and
the `CAP_DAC_READ_SEARCH` capability to permit system-wide
read access while blocking write access does not work.
Experiments have shown that `dsmc` verifies access permissions
for each file before attempting to open it for reading.
Hence `dsmc` refuses to copy files where the file permission
mode blocks read access -- even if process capabilities
would allow it to proceed irrespective of permissions.
c5effcaaea