refactor command mode detection

refactor command mode detection

Splits the check for a command's ability to run into a helper.

This also fixes a bug whereby commands that specified COMMAND_EXEC
were allowed to run during the configuration stage.  This allowed
problematic commands to be called before 'init', defeating the intention
of specifying that command mode.  With this change, the run_command()
helper denies access to handlers that should run only after 'init'
during the configuration stage.