config: generate more relevant config for FreeBSD and OpenBSD

This change makes config generator make more relevant config files on FreeBSD and OpenBSD. (Actually it changes audio setup and removes `epoll` from `poll_method` setting if system has no epoll support.)

There are two changes I also try to push here:
1. `#if defined` tests are sorted alphabetically so that it is easier to navigate through them;
2. `oss` module is not listed in config on Linux by default.