generator 90services-select fails if restricted and multiverse are both disabled

[Sgifford]

Debugging a problem with a preseeded custom installation of Ubuntu Precise Pangolin, I saw this error report in my installation log:

  apt-setup: warning: /usr/lib/apt-setup/generators/90services-select returned error code 1; discarding output

Troubleshooting the error, I found that it occurred when running "disable_service partner".

The script runs with "-e" mode enabled, which causes it to immediately exit if any command or function exits/returns nonzero.

The "disable_service" function returns nonzero if the service was not actually removed, so when "disable_service partner" is run and "partner" was not previously enabled, it returns nonzero, and the script aborts.

It looks like the return value from "disable_service" and "enable_service" is not used anywhere, so the easiest fix would be to simply never return nonzero from those functions.