Revert "Add a run-as-user service wrapper"

We're going to try to avoid modifying passwd as much as possible, see
discussion at
https://github.com/zalora/defnix/commit/34dd080d4f40e77655357e126b07e8bd04131a06#commitcomment-8885919.

This reverts commit 1870adb60dd8107883ea786f9b1757fc99c6e3fa.
Revert "Add an add-user package suitable for use in an initializer"

We're going to try to avoid modifying passwd as much as possible, see
discussion at
https://github.com/zalora/defnix/commit/34dd080d4f40e77655357e126b07e8bd04131a06#commitcomment-8885919.

This reverts commit 34dd080d4f40e77655357e126b07e8bd04131a06.
execve: enable specifying private bind mounts
Add inetd emulation pkg
Add sshd service
Fix typo in readiness notification service
Fix generate-nixos-config for on-demand services