Running haproxy with "DynamicUser = true" doesn't really work, since
it prohibits specifying a TLS certificate bundle with limited
permissions. This revives the haproxy user and group, but makes them
dynamically allocated by NixOS, rather than statically allocated. It
also adds options to specify which user and group haproxy runs as.
HAProxy fails to start with the default 'config'. Better disable it and
assert that the user provides a suitable 'config'. (AFAICS, there cannot
really be a default config file for HAProxy.)
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.
