funk: warn on OS-specific commands

[mcandre]

Warn on OS-specific commands, such as invoking package managers, e.g. pacman.

Recommend to either suffix the filename to include the OS name, e.g. Arch. Or else replace the command with a universal provisioning playbook, e.g. Ansible.

Detect when multiple OS's are involved, so that polyglot shell scripts are ignored.

There are so many exceptions to this rule, that we should keep this warning turned off by default.