bash: Build new PATH the smart way

[ermo]

Summary The goal with this PR is to ensure that any existing user-set PATH is respected to the widest extent possible. Any elements of the default PATH which have not already been added, will be added to the new PATH in (what I hope to be) a sane way.

In the case of no user-set PATH, prepare a sane default PATH.

I have left the debug statements in for now to provide visibility to the various test cases. Naturally, these debug statements will need to disappear before the commit is merged to main.

Test Plan

Open a new virtual terminal with the new 10-path.sh file in place in /usr/share/defaults/etc/profile.d/

Checklist

• [x] Package was built and tested against unstable