CLEANUP: vars: factor out common code from vars_get_by_{desc,name}

CLEANUP: vars: factor out common code from vars_get_by_{desc,name}

The two functions vars_get_by_name() and vars_get_by_scope() perform
almost the same operations except that they differ from the way the
name and scope are retrieved. The second part in common is more
complex and involves locking, so better factor this one out into a
new function.

There is no other change than refactoring.