Be a good shell citizen

[spl]

The point of this project is to hook into parts of the Bash system to allow functions to be run. However, the Bash system is complex, users of this project may have complex configurations, and there may be other parts trying to do similar things as this project. In those cases, it would be better for users if we do not assume that we are the only ones doing anything interesting.

Concretely, this means:

1. We should not replace the PROMPT_COMMAND if it exists and is not empty.
2. We should not clear the DEBUG trap (i.e. via trap DEBUG) if it is already set.

However, since the user may not be away of these conflicts, we should warn them when they occur.

[spl]

Currently, we're checking for duplicate sourcing at the beginning of the file. But it's fine if we source the functions again, esp. now that global initializations are removed (#18). In particular, we want to re-source the functions if they've been updated. However, it may not be fine if we change the PROMPT_COMMAND and DEBUG trap.

What we can do is check if the new values would be different from the old, give a warning if they are, and don't do anything.

That is, what's different from the description above is to not just check if the values are empty but to check if they differ from what we would set.