Refactor .shared_env OS tests and add descriptions.
Refactor .shared_env OS tests and add descriptions.

- Remove `return 1` statements since they are redundant and are just
  repreating what the test commands already return.
- Add a 'linux' value to `get_os`, remove the redunant return value
  test, use a clearer `if/then` construct, and `return 0` when the
  system is found so the loop doesn't keep going and `return 1` on
  failed test after the successful one.

alphabetum/dotfiles,alphabetum/dotfiles,alphabetum/dotfiles,alphabetum/dotfiles