manager: read ~/.config/systemd/user.conf in user mode

manager: read ~/.config/systemd/user.conf in user mode

This changes the paths we read user manager config from in two ways:

- split-usr-root paths are dropped. The user manager is a poster boy for
  non-early-boot, so reading dropins only from /usr is appropriate.

- we look at ~/.config/systemd/user.conf. Users should be allowed to override
  their own config.

As user managers become more and more used, it becomes more important for users
to customize their own daemon. By reading from ~/.config, this is possible
without privileges.