OpenVPN is enabled implicitly when configuring 'services.openvpn.servers', so the
"enable" option is meaningless since b2910df04e4ec3e707ad08074bd128dedf9b2c23.
Closes https://github.com/NixOS/nixpkgs/issues/9764.
This causes OpenVPN services to reach the "active" state when the VPN
connection is up (i.e., after OpenVPN prints "Initialization Sequence
Completed"). This allows units to be ordered correctly after openvpn-*
units, and makes systemctl present a password prompt:
$ start openvpn-foo
Enter Private Key Password: *************
(I first tried to implement this by calling "systemd-notify --ready"
from the "up" script, but systemd-notify is not reliable.)
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
