Fix commands, the easy way.

Don't use pluginhook to run the commands hook, just run all the scripts
in a loop.
Don't explicitly specify streams to docker run.

Now that "docker run" always has an stdin, we don't need to explicitly
specify stdout and stderr.
Remove known issues and add note about ForceTTY

These issues are fixed by this PR.
Close stdin before running the backup-export hooks.

This avoids hanging when we're not in a pty.
Merge pull request #344 from plietar/commandfix

Fix commands, the easy way.