Preview udev rules before installing them and install with mode 644

[Copilot]

seabreeze_os_setup installed the udev rules without ever showing what it was about to write to /etc/udev/rules.d/, and offered no path for installing them by hand. It also copied the rules straight from the 0600 temporary download file, so the installed rules ended up root-only readable.

Changes

• Preview: the contents of the rules file (downloaded from GitHub or passed on the command line) are printed along with the target path, right before the confirmation prompt.
• Manual install: declining the prompt now prints the commands needed to install the previewed rules by hand instead of silently exiting.
• Permissions: sudo cp replaced with sudo install -m 644, so the rules are world readable regardless of the source file mode. A non-zero return code now exits with 1 instead of printing Success.
• Docs: os_support/readme.md and CHANGELOG updated to describe the preview/confirm/644 behavior.
• Tests: new tests/test_os_setup.py covering the preview output, the install -m 644 invocation, the declined path, and a failing install.

Example

$ seabreeze_os_setup
The following udev rules will be installed as /etc/udev/rules.d/10-oceanoptics.rules:
  # udev rules file for Ocean Optics, Inc. spectrometers
  # ====================================================
  ...
  LABEL="oceanoptics_rules_end"
Install udev rules? [y/n] n
To install the rules manually, copy the rules shown above to
/etc/udev/rules.d/10-oceanoptics.rules and run:

  sudo chmod 644 /etc/udev/rules.d/10-oceanoptics.rules
  sudo udevadm control --reload-rules

• Fixes print udev rules before installing them/allow manual installation of rules #300