Configuring Subversion to use system users/passwords
The question mentions basic/digest auth. First, note that basic/digest auth refers to the protocol between server and browser to communicate the user/password (on Apache 2.2, using authentication front-end modules such as mod_auth_digest and mod_auth_basic).
Behind the scenes, Apache can use a number of different sources of user/password information to check against. mod_authn_file which uses a "password file" is a common one, and it sounds as though that's what you're referring to.
To access the system user/password on a Linux box, Apache project itself doesn't provide any authentication back-end modules, but there are 3rd-party ones available. See this article which may be helpful: