Made documentation less redundant

- Removed redundant / unnecessarily verbose text from documentation.
- Various minor changes for readability and consistency.
Always set EXIT trap in main to ensure sane TTY
Check for existence of volume image in "mount"
Explicit permissions for install target, bugfix

- Added explicit read and write permissions for all users to the uuli
  script and explicit read for all users to the configuration file.
- Fixed typo in uninstall target.
Changed disk image naming convention

- All disk images are prefixed with a "." to make it possible to
  immediately auto-complete folder names in a shell without conflicts
  between the mount point and corresponding disk image.
Validate directory and disk image permissions

- User directory permissions are validated before the script executes
  any commands, and disk image permissions are checked prior to mounting
  any images.