perl5db: fixes for issues related to overloads

[haarg]

When perl calls DB::sub in the debugger, it sets $DB::sub to the sub
being called. If we trigger any other subs to get called somewhere
inside DB::sub, this could cause DB::sub to be called again, overwriting
the global. Perl won't call DB::sub for any sub calls within the DB
namespace, but we could inadvertently trigger this via a sub override or
magic, like overloads.

Create a copy of the global value in a lexical to use inside the sub, to
avoid it ever getting overwritten.

DB::sub can be called by perl giving it either a name (if it can be
determined) or a code ref. There is special handling for AUTOLOAD subs
and threads::new. This could only be happen when given the name of the
sub, so there is no need to do these checks if given a reference.
Additionally, a reference could be an object, which could have
overloads. Those overloads could fail or otherwise complicate the normal
operation of DB::sub.

Add a ref check to the sub given to DB::sub to avoid these issues.

Fixes #23486

[tonycoz]

Any chance of tests?

There's a framework for testing the debugger that's usable from lib/perl5db.t.