Preserved DEBUG traps should be invoked every time, not just on preexec

[dimo414]

A coworker pointed out that that the fix to #39 doesn't properly preserve the behavior of the DEBUG trap, because preexec only triggers on interactive prompts, whereas before installation the command was triggered on every DEBUG event.

I think this can be fixed by simply moving the old DEBUG trap out of preexec_functions and instead eval-ing it at the beginning of __bp_preexec_invoke_exec - before the short-circuiting checks are made.

The only problem with this is it makes it difficult for someone to clean up the old trap - it's no longer in the trap itself nor in the preexec / preexec_functions, but some other variable. It's simple enough to document that (e.g. "The old DEBUG trap will be preserved in a bp_prior_debug_trap variable") but that widens the API surface and might be abused.

I'll prepare a patch, but I wanted to get some opinions on the approach first.

[rcaloras]

@dimo414 interesting observation. Since the original implementation simply overwrote the existing DEBUG trap I think anything is really an improvement here :)

I'm in favor of having a variable preserving the contents of the prior debug trap and invoked how you spec'd. Although it widens the API it's definitely a better experience. It does make things a little more complicated, but if we want to support legit preservation it's worth the compromise 👍

[rcaloras]

Assigning over! As always thanks for the contributions 💯 🥇