Switch several caller() invocations to explicit CORE::caller()

Switch several caller() invocations to explicit CORE::caller()

This not only fixes an obscure test failure due to an older Sub::Uplevel
(it would be too obnoxious to bump the dep just for the test case), but also
makes the entire codebase more robust in light of possible rogue/incomplete
caller() overrides

This was not a simple s/// job - each change was manually evaluated before
carrying out

Also while at it - fix the utterly annoying *UNKNOWN* eception site-marker:
it does not add any information and only confuses things. The heuristics in
Carp::Skip is supposed to be much much clearer, need to finish its tests...