Automatic detection of command prompt for specific agents

This task is hard in general but for fence devices that uses command line interface only, it should be doable without major problems.

The issue names no files, tests, or specific agents. Start by identifying the fence agents that use only a command-line interface and determine how command-prompt detection should work for them; done means those agents can detect their command prompt automatically without disrupting existing fencing behavior.

Written by the indexing model from the issue text.