Make global_symbol_searcher::filenames private

Make global_symbol_searcher::filenames private

This patch renames global_symbol_searcher::filenames and makes it
private, adding a new method to append a filename to the vector.  This
also cleans up memory management here, removing an alloca from rbreak,
and removing a somewhat ugly SCOPE_EXIT from the Python code, in favor
of having global_symbol_searcher manage the memory itself.

Regression tested on x86-64 Fedora 38.