[perl #119799] Set breakpoints without *DB::dbline

[perl #119799] Set breakpoints without *DB::dbline

The elements of the %{"_<..."} hashes (where ‘...’ is the filename),
whose keys are line numbers, are used to set breakpoints on the given
lines.  The corresponding @{"_<..."} array contains the actual lines
of source code.

%{"_<..."} actually acts on the array of lines that @DB::dbline is
aliased to.  The assumption is that *DB::dbline = *{"_<..."} will have
taken place first.  Hence, all %{"_<..."} hashes are the same, when it
comes to writing to keys.

It is more useful for each %{"_<..."} hash to set breakpoints on its
corresponding file’s lines regardless of whether @DB::dbline has been
aliased, so that is what this commit does.

Each hash’s mg_obj pointer in its dbfile magic now points to the
array, and magic_setdbline uses it instead of PL_DBline.