gdb: use make_scoped_restore to restore gdbpy_current_objfile

gdb: use make_scoped_restore to restore gdbpy_current_objfile

The current mechanism by which the Python gdb.current_objfile is
maintained does not allow for nested auto-load events.  It is assumed
that once an auto-load script has finished loading then the current
objfile should be set back to NULL.  In a nested situation, we should
be restoring the previous value.

We already have an RAII class to handle save/restore type behaviour,
so lets just switch to use that.

The test is a little contrived, but is simple enough, and triggers the
bug.  The real use case might involve the auto-load script calling
functions (either in the just-loaded object file, or in the main
executable), which in turn trigger further auto-loads to occur.

gdb/ChangeLog:

	* python/python.c (gdbpy_source_objfile_script): Use
	make_scoped_restore to restore gdbpy_current_objfile.
	(gdbpy_execute_objfile_script): Likewise.

gdb/testsuite/ChangeLog:

	* gdb.python/py-auto-load-chaining-f1.c: New file.
	* gdb.python/py-auto-load-chaining-f1.o-gdb.py: New file.
	* gdb.python/py-auto-load-chaining-f2.c: New file.
	* gdb.python/py-auto-load-chaining-f2.o-gdb.py: New file.
	* gdb.python/py-auto-load-chaining.c: New file.
	* gdb.python/py-auto-load-chaining.exp: New file.