Add enable flags for each db (passwd,group,host,services,netgroup)

This really is just pulling together the work done in PR #4 and #5 (and PR #7, but less important), getting some unit testing done for #4, and setting the netgroup DB stuff adding in #5 to be off by default as having the netgroup db stanza breaks on some platforms (EL5 and EL6 with an old enough nscd, for example).