lualoader: Expose loader.parse and add cli.parse_and_execute

[kevans]

Summary

This will be used for scenarios where the command to execute is coming in via the environment (from, for example, loader.conf(5)) and is thus not necessarily trusted.

parse_and_execute will immediately be used for handling module_{before,after,error} as well as menu_timeout_command. We still want to offer these variables the ability to execute Lua-intercepted loader commands, but we don't want them to be able to execute arbitrary Lua.

[kevans]

I should probably add a note to 'cli' that most of these functions will violate our standard naming convention... loader commands tend to use hyphens or underscore to separate words, so functions that may be executed at the loader prompt should follow suit.